Viruses
Rotavirus
Virus classification
Group: I–VII
Groups
I: dsDNA viruses II: ssDNA viruses III: dsRNA viruses IV: (+)ssRNA viruses V: (-)ssRNA viruses VI: ssRNA-RT viruses VII: dsDNA-RT viruses

A virus (from the Latin virus meaning "toxin" or "poison"), is a sub-microscopic infectious agent that is unable to grow or reproduce outside a host cell. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. A toxin ( Greek:, toxikon, lit (poison for use on arrows is a Poisonous substance produced by living cells or organisms that is active at very low In the context of Biology, poisons are substances that can cause damage, Illness, or Death to Organisms usually by The optical microscope, often referred to as the "light microscope" is a type of Microscope which uses Visible light and a system of lenses to In Biology, a host is an organism that harbors a Virus or Parasite, or a mutual or Commensal Symbiont, typically providing nourishment The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called Each viral particle, or virion, consists of genetic material, DNA or RNA, within a protective protein coat called a capsid. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units For the leaf bug see Miridae. A capsid is the protein shell of a virus. The capsid shape varies from simple helical and icosahedral (polyhedral or near-spherical) forms, to more complex structures with tails or an envelope. Many Viruses (eg influenza and many animal viruses have viral envelopes covering their protein Capsids The envelopes are typically derived from portions of Viruses infect cellular life forms and are grouped into animal, plant and bacterial types, according to the type of host infected.

Biologists debate whether or not viruses are living organisms. Foundations of modern biology There are five unifying principles Some consider them non-living as they do not meet all the criteria used in the common definitions of life. Life is a state that distinguishes Organisms from non-living objects such as non-life and dead organisms being manifested by growth through Metabolism For example, unlike most organisms, viruses do not have cells. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called However, viruses have genes and evolve by natural selection. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of Others have described them as organisms at the edge of life. Viral infections in human and animal hosts usually result in an immune response and disease. A disease is an abnormal condition of an organism that impairs bodily functions and can be deadly Often, a virus is completely eliminated by the immune system. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. In modern usage an antibiotic is a Chemotherapeutic agent with activity against Microorganisms such as Bacteria, fungi or Protozoa Antiviral drugs are a class of Medication used specifically for treating viral Infections Like Antibiotics for Bacteria, specific antivirals Vaccines that produce lifelong immunity can prevent viral infections. A vaccine is a biological preparation which is used to establish or improve immunity to a particular disease Immunity is a material term that describes a state of having sufficient biological defenses to avoid Infection, Disease, or other unwanted biological invasion

Etymology

The word is from the Latin virus referring to poison and other noxious substances, first used in English in 1392. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. In the context of Biology, poisons are substances that can cause damage, Illness, or Death to Organisms usually by ^[1] Virulent, from Latin virulentus, "poisonous", dates to 1400. ^[2] A meaning of "agent that causes infectious disease" is first recorded in 1728,^[1] before the discovery of viruses by the Russian-Ukrainian biologist Dmitry Ivanovsky in 1892. The Russian people (Русские— Russkie) are an East Slavic Ethnic group, primarily living in Russia and neighboring countries Ukrainians (Українці Ukrayintsi,) are an East Slavic Ethnic group primarily living in Ukraine, or more broadly— Citizens A biologist is a Scientist devoted to and producing results in Biology through the study of Organisms Typically biologists study organisms and their relationship The adjective viral dates to 1948. ^[3] Today, virus is used to describe the biological viruses discussed above and as a metaphor for other parasitically-reproducing things, such as memes or computer viruses (since 1972). Metaphor (from the Greek: μεταφορά - metaphora, meaning "transfer" is language that directly compares seemingly unrelated subjects A meme (miːm consists of any idea or behavior that can pass from one person to another by learning or imitation A computer virus is a Computer program that can copy itself and infect a computer without permission or knowledge of the user ^[2] The term virion is also used to refer to a single infective viral particle. The English plural form of virus is viruses; see Plural of virus. In the English language, the plural of virus is viruses. In reference to a Computer virus, the plural is often believed to be virii or

Discovery of viruses

Viral diseases such as rabies, yellow fever and smallpox have affected humans for centuries. Rabies (from rabies “madness rage fury” Also known as “ hydrophobia ” is a viral Zoonotic neuroinvasive disease that Yellow fever (also called yellow jack, black vomit or sometimes American Plague) is an acute viral disease Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor. There is hieroglyphical evidence of polio in ancient Egyptian medicine,^[4] though the cause of this disease was unknown at the time. Poliomyelitis, often called polio or infantile paralysis, is an acute viral Infectious disease spread from person to person primarily via Ancient Egyptian Medicine refers to the practices of healing common in Ancient Egypt from Circa 3300 BC until the Persian In the 10th century, Muhammad ibn Zakarīya Rāzi (Rhazes) wrote the Treatise on Smallpox and Measles, in which he gave the first clear descriptions of smallpox and measles. Measles (rubeola is a Disease caused by a virus specifically a Paramyxovirus of the genus Morbillivirus. ^[5]

In 1717, Mary Montagu, the wife of an English ambassador to the Ottoman Empire, observed local women inoculating their children against smallpox. The Lady Mary Wortley Montagu ( 26 May 1689 &ndash 21 August 1762) was an English aristocrat and writer The Ottoman Empire (1299–1923 ( Old Ottoman Turkish: دولتْ علیّه عثمانیّه Devlet-i Âliye-yi Osmâniyye, Late Ottoman and Modern Turkish Inoculation is the placement of something to where it will grow or reproduce and is most commonly used in respect of the introduction of a serum Vaccine, or antigenic substance Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor. ^[6] In the late 18th century, Edward Jenner observed and studied Miss Sarah Nelmes, a milkmaid who had previously caught cowpox and was found to be immune to smallpox, a similar, but devastating virus. Edward Jenner, FRS, ( May 17 1749 – January 26 1823) was an English scientist who studied his natural surroundings in Berkeley Cowpox is a Disease of the skin that is caused by a Virus known as the Cowpox virus. Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor. Jenner developed the smallpox vaccine based on these findings. A vaccine is a biological preparation which is used to establish or improve immunity to a particular disease After lengthy vaccination campaigns, the World Health Organization (WHO) certified the eradication of smallpox in 1979. Vaccination is the administration of Antigenic material (the Vaccine) to produce immunity to a disease Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor.

In the late 19th century, Charles Chamberland developed a porcelain filter with pores small enough to remove cultured bacteria from their culture medium. Charles Chamberland ( March 12, 1851 - May 2, 1908) was a French Microbiologist from Chilly-le-Vignoble in the department of Jura ^[7] Dimitri Ivanovski used this filter to study an infection of tobacco plants, now known as tobacco mosaic virus. Tobacco mosaic virus (TMV is an RNA virus that infects Plants especially Tobacco and other members of the family Solanaceae. He passed crushed leaf extracts of infected tobacco plants through the filter, then used the filtered extracts to infect other plants, thereby proving that the infectious agent was not a bacterium. Similar experiments were performed by several other researchers, with similar results. These experiments showed that viruses are orders of magnitude smaller than bacteria. An order of magnitude is the class of scale or magnitude of any amount where each class contains values of a fixed ratio to the class preceding it The term virus was coined by the Dutch microbiologist Martinus Beijerinck, who showed, using methods based on the work of Ivanovski, that tobacco mosaic disease is caused by something smaller than a bacterium. Martinus Willem Beijerinck ( March 16, 1851 - January 1, 1931) was a Dutch Microbiologist and Botanist He coined the Latin phrase "contagium vivum fluidum" (which means "soluble living germ") as the first idea of the virus. ^[8] The first human virus identified was Yellow Fever virus. Yellow fever (also called yellow jack, black vomit or sometimes American Plague) is an acute viral disease

In the early 20th century, Frederick Twort discovered that bacteria could be infected by viruses. Frederick William Twort (1877-1950 was an English Bacteriologist. ^[9] Felix d'Herelle, working independently, showed that a preparation of viruses caused areas of cellular death on thin cell cultures spread on agar. Félix d'Herelle ( April 25, 1873 &ndash February 22, 1949) was a French-Canadian microbiologist, the co-discoverer of Cell culture is the process by which prokaryotic, or eukaryotic cells are grown under controlled conditions Agar or agar agar is a Gelatinous substance derived from Seaweed. Counting the dead areas allowed him to estimate the original number of viruses in the suspension. The invention of electron microscopy provided the first look at viruses. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image In 1935, Wendell Stanley crystallized the tobacco mosaic virus and found it to be mostly protein. Wendell Meredith Stanley ( August 16, 1904 &ndash June 15, 1971) was an American biochemist, virologist Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl ^[10] A short time later, the virus was separated into protein and nucleic acid parts. A nucleic acid is a Macromolecule composed of chains of monomeric Nucleotides In Biochemistry these Molecules carry Genetic information ^[11][12] In 1939, Max Delbrück and E. Max Ludwig Henning Delbrück ( September 4, 1906 &ndash March 9, 1981) was a German-American Biophysicist and Nobel L. Ellis demonstrated that, in contrast to cellular organisms, bacteriophage reproduce in "one step", rather than exponentially. ^[13]

A major problem for early virologists was the inability to propagate viruses on sterile culture media, as is done with cellular microorganisms. This limitation required medical virologists to infect living animals with infectious material, which is dangerous. The first breakthrough came in 1931, when Ernest William Goodpasture demonstrated the growth of influenza and several other viruses in fertile chicken eggs. Dr Ernest William Goodpasture ( October 17, 1886 &ndash September 20, 1960) was an American Pathologist and Physician ^[14] However, some viruses would not grow in chicken eggs, and a more flexible technique was needed for propagation of viruses. The solution came in 1949 when John Franklin Enders, Thomas H. Weller and Frederick Chapman Robbins together developed a technique to grow the polio virus in cultures of living animal cells. John Franklin Enders ( February 10 1897 &ndash September 8 1985) was an American medical scientist and Nobel laureate. Thomas Huckle Weller ( June 15, 1915 &ndash August 23, 2008) was an American Virologist. Frederick Chapman Robbins ( August 25, 1916 &ndash August 4, 2003) was an American pediatrician and virologist Poliovirus, the causative agent of Poliomyelitis, is a human Enterovirus and member of the family of Picornaviridae. ^[15] Their methods have since been extended and applied to the growth of viruses and other infectious agents that do not grow on sterile culture media.

Origins

The origin of modern viruses is not entirely clear. It may be that no single mechanism can account for their origin. ^[16] They do not fossilize well, so molecular techniques have been the most useful means of hypothesising how they arose. FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. Molecular biology is the study of Biology at a molecular level ^[17] Research in microfossil identification and molecular biology may yet discern fossil evidence dating to the Archean or Proterozoic eons. Micropaleontology (also sometimes spelled as micropalaeontology) is that branch of Paleontology which studies microfossils The Proterozoic (ˌproʊtərəˈzoʊɪk is a geological eon representing a period before the first abundant complex life on Earth. Two main hypotheses currently exist. ^[18]

Small viruses with only a few genes may be runaway stretches of nucleic acid originating from the genome of a living organism. Their genetic material could have been derived from transferable genetic elements such as plasmids or transposons, that move within, leave, and enter genomes. A plasmid is an extra-chromosomal DNA molecule separate from the chromosomal DNA which is capable of replicating independently of the chromosomal DNA Transposons are sequences of DNA that can move around to different positions within the Genome of a single cell, a process called transposition New viruses are emerging de novo and therefore, it is not always the case that viruses have ancestors. ^[19]

Viruses with larger genomes, such as poxviruses, may have once been small cells that parasitized larger host cells. Poxviruses (members of the family Poxviridae) are viruses that can as a family infect both Vertebrate and Invertebrate animals Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as retrograde-evolution or reverse-evolution. The bacteria Rickettsia and Chlamydia are living cells that, like viruses, can only reproduce inside host cells. Rickettsia is a Genus of motile, Gram-negative, non-sporeforming, highly Pleomorphic bacteria that can present Chlamydia is a Genus of Bacteria in the family Chlamydiaceae, order Chlamydiales, class and phylum Chlamydiae They lend credence to the streamlining hypothesis, as their parasitic lifestyle is likely to have led to the loss of genes that enabled them to survive outside a host cell.

It is possible that viruses represent a primitive form of self replicating DNA and are a precursor to life as it is currently defined. ^[20] Other infectious particles which are even simpler in structure than viruses include viroids, satellites, and prions. Viroids are Plant Pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary circular single-stranded A Satellite is a Subviral agent composed of Nucleic acid that depends on the Coinfection of a host cell with a helper or master Virus for their A prion (ˈpriːɒn is thought to be an infectious agent that according to current scientific consensus is comprised entirely of a propagated, mis-folded

Classification

Main article: Virus classification

In taxonomy, the classification of viruses is difficult owing to the lack of a fossil record and the dispute over whether they are living or non-living. Virus classification involves naming and placing Viruses into a taxonomic system Taxonomy is the practice and science of classification The word comes from the Greek, taxis (meaning 'order' 'arrangement' and, nomos ^[21][22] They do not fit easily into any of the domains of biological classification, and classification begins at the family rank. In biological Taxonomy, a domain (also superregnum, superkingdom, or empire) is the highest Taxonomic rank of Organisms In Biological classification, family ( Latin However, the domain name of Acytota (without cells) has been suggested. Non-cellular life is Life that exists without cells This term usually is applied to earth-based life and presumes the Phylogenetic classification This would place viruses on a par with the other domains of Eubacteria, Archaea, and Eukarya. The Bacteria ( singular: bacterium) are a large group of unicellular Microorganisms Typically a few Micrometres in length bacteria have Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex Not all families are currently classified into orders, nor all genera classified into families.

In 1962, André Lwoff, Robert Horne, and Paul Tournier were the first to develop a means of virus classification, based on the Linnaean hierarchical system. André Michel Lwoff ( May 8, 1902 &ndash September 30, 1994) was a French Microbiologist. Robert Horne may be Robert Horne (bishop (1513/15-1579 bishop of Winchester Robert Stevenson Horne 1st Viscount Horne (1871&ndash1940 Paul Tournier ( May 12, 1898 - October 7, 1986) was a Swiss Physician and Author who had acquired a worldwide Linnaean taxonomy is a method of classifying living things originally devised by (and named for Carolus Linnaeus, although it has changed considerably since his time ^[23] This system based classification on phylum, class, order, family, genus, and species. A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class. A class is the Taxonomic rank in the Biological classification of organisms in Biology below phylum and above order. This article is about the taxonomic rank for the sequence of species in a taxonomic list see Taxonomic order In scientific classification used In Biological classification, family ( Latin A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. Viruses were grouped according to their shared properties (not of their hosts) and the type of nucleic acid forming their genomes. ^[24] Following this initial system, a few modifications were made and the International Committee on Taxonomy of Viruses was developed (ICTV). The International Committee on Taxonomy of Viruses ( ICTV) is a committee which authorizes and organizes the taxonomic classification of Viruses They have

ICTV classification

The International Committee on Taxonomy of Viruses (ICTV) developed the current classification system and put in place guidelines that put a greater weighting on certain virus properties to maintain family uniformity. The International Committee on Taxonomy of Viruses ( ICTV) is a committee which authorizes and organizes the taxonomic classification of Viruses They have A universal system for classifying viruses, and a unified taxonomy, has been established since 1966. In determining order, taxonomists should consider the type of nucleic acid present, whether the nucleic acid is single- or double-stranded, and the presence or absence of an envelope. Many Viruses (eg influenza and many animal viruses have viral envelopes covering their protein Capsids The envelopes are typically derived from portions of After these three main properties, other characteristics can be considered: the type of host, the capsid shape, immunological properties and the type of disease it causes. The system makes use of a series of ranked taxons. A taxon (plural taxa) or taxonomic unit, is a name designating an organism or a group of Organisms In Biological nomenclature according to The general structure is as follows:

Order (-virales)

Family (-viridae)

Subfamily (-virinae)

Genus (-virus)

Species (-virus)

The recognition of orders is very recent; to date, only three have been named, and most families remain unplaced. This article is about the taxonomic rank for the sequence of species in a taxonomic list see Taxonomic order In scientific classification used In Biological classification, family ( Latin In Biological classification, family ( Latin A genus (plural genera from Γένος Latin genus "descent family type gender" is a low-level Taxonomic In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. The committee does not formally distinguish between subspecies, strains, and isolates. In total there are three orders, 56 families, nine subfamilies, and 233 genera. ICTV recognizes about 1,550 virus species, but about 30,000 virus strains and isolates are being tracked by virologists. ^[25]

The Nobel Prize-winning biologist David Baltimore devised the Baltimore classification system. The Nobel Prize (Nobelpriset (Nobelprisen is a Swedish prize established in the 1895 will of Swedish chemist Alfred Nobel; it was first awarded in Peace, Literature David Baltimore (born March 7, 1938) is an American biologist Virus classification involves naming and placing Viruses into a taxonomic system ^[26][27] The ICTV classification system is used in conjunction with the Baltimore classification system in modern virus classification. ^[28][29][30]

Baltimore Classification

The Baltimore Classification of viruses is based on the method of viral mRNA synthesis. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product

Main article: Baltimore classification

The Baltimore classification of viruses is based on the mechanism of mRNA production. The Baltimore classification is a Virus classification system which groups Viruses into families depending on their type of Genome ( DNA Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Viruses must generate positive strand mRNAs from their genomes to produce proteins and replicate themselves, but different mechanisms are used to achieve this in each virus family. This classification places viruses into seven groups:

• I: dsDNA viruses (e. A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. g. Adenoviruses, Herpesviruses, Poxviruses)

• II: ssDNA viruses (+)sense DNA (e. Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Poxviruses (members of the family Poxviridae) are viruses that can as a family infect both Vertebrate and Invertebrate animals A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. g. Parvoviruses)

• III: dsRNA viruses (e. Parvovirus, commonly abbreviated to parvo, is a Genus of the Parvoviridae family linear non-segmented single stranded DNA viruses Double-stranded ( ds) RNA viruses are a diverse group of Viruses that vary widely in host range (humans animals plants Fungi, and g. Reoviruses)

• IV: (+)ssRNA viruses (+)sense RNA (e. Reoviridae is a family of Viruses that can affect the Gastrointestinal system (such as Rotavirus) and Respiratory tract An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. g. Picornaviruses, Togaviruses)

• V: (-)ssRNA viruses (-)sense RNA (e. A Picornavirus is a Virus belonging to the family Picornaviridae. The Togaviridae are a family of Viruses including the following genera Genus Alphavirus; type species Sindbis virus An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. g. Orthomyxoviruses, Rhabdoviruses)

• VI: ssRNA-RT viruses (+)sense RNA with DNA intermediate in life-cycle (e. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that Rhabdoviruses are Viruses belonging to the family Rhabdoviridae, which is in the order Mononegavirales. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template g. Retroviruses)

• VII: dsDNA-RT viruses (e. A retrovirus is any Virus belonging to the viral family Retroviridae. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template g. Hepadnaviruses)

As an example of viral classification, the chicken pox virus, Varicella zoster (VZV), belongs to family Herpesviridae, subfamily Alphaherpesvirinae and genus Varicellovirus. Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals Chickenpox is a highly contagious illness caused by primary infection with Varicella zoster virus (VZV Varicella zoster virus ( VZV) is one of eight herpes viruses known to infect humans (and other vertebrates The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Alphaherpesvirinae is a subfamily of Herpesviridae primarily distinguished by reproducing more quickly than other subfamilies of Herpesviridae Varicellovirus (var′i-sel′ō-vi′rŭs is a genus of Alphaherpesvirinae. It remains unranked in terms of order. VZV is in Group I of the Baltimore Classification because it is a dsDNA virus that does not use reverse transcriptase. In Biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase Enzyme that transcribes

Structure

A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. A nucleic acid is a Macromolecule composed of chains of monomeric Nucleotides In Biochemistry these Molecules carry Genetic information Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl For the leaf bug see Miridae. A capsid is the protein shell of a virus. Viruses can have a lipid "envelope" derived from the host cell membrane. Lipids are broadly defined as any fat- Soluble ( lipophilic) naturally-occurring Molecule, such as fats oils waxes cholesterol sterols fat-soluble The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer A capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological and antigenic distinction. In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism ^[31][32] Virally coded protein subunits will self-assemble to form a capsid, generally requiring the presence of the virus genome. However, complex viruses code for proteins which assist in the construction of their capsid. ^[18] Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. A nucleoprotein is any Protein which is structurally associated with Nucleic acid (either DNA or RNA)

In general, there are four main morphological virus types:

Helical viruses
Diagram of a helical capsid	Helical capsids are composed of a single type of subunit stacked around a central axis to form a helical structure which may have a central cavity, or hollow tube. This arrangement results in rod-shaped or filamentous virions: these can be anything from short and highly rigid, to long and very flexible. The genetic material, generally single-stranded RNA, but ssDNA in some cases, is bound into the protein helix, by interactions between the negatively-charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of protomers. The well-studied Tobacco mosaic virus is an example of a helical virus. Tobacco mosaic virus (TMV is an RNA virus that infects Plants especially Tobacco and other members of the family Solanaceae.
Icosahedron viruses
Electron micrograph of icosahedral virions	Icosahedral capsid symmetry results in a spherical appearance of viruses at low magnification but actually consists of capsomers arranged in a regular geometrical pattern, similar to a soccer ball, hence they are not truly "spherical". Association football, more commonly known as football or soccer, is a Team sport played between two teams of eleven players and is widely considered Capsomers are ring shaped constructed from five to six copies of protomers. These associate via non-covalent bonding to enclose the viral nucleic acid, though generally less intimately than helical capsids, and may involve one or more protomers. A chemical bond is the physical process responsible for the attractive interactions between Atoms and Molecules and which confers stability to diatomic and polyatomic Icosahedral architecture was employed by R. Buckminster Fuller in his geodesic dome, and is the most efficient way of creating an enclosed robust structure from multiple copies of a single protein. Richard Buckminster “Bucky” Fuller ( July 12, 1895 &ndash July 1, 1983) was an American Architect, Author A geodesic dome is an almost spherical shell structure based on a network of Great circles ( Geodesics lying approximately on the surface of a Sphere The number of proteins required to form a spherical virus capsid is denoted by the T-number,[33] where 60×t proteins are necessary. In the case of the hepatitis B virus the T-number is 4, and 240 proteins assemble to form the capsid.
Enveloped viruses
Herpes zoster virus	Viruses are able to envelope themselves in a modified form of one of the cell membranes either the outer membrane surrounding an infected host cell, or from internal membranes such as nuclear membrane or endoplasmic reticulum, thus gaining an outer lipid bilayer known as a viral envelope. The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Many Viruses (eg influenza and many animal viruses have viral envelopes covering their protein Capsids The envelopes are typically derived from portions of This membrane is studded with proteins coded for by the viral genome and host genome; however the lipid membrane itself and any carbohydrates present are entirely host-coded. The Influenza virus and HIV use this strategy. The viral envelope can give a virion a few distinct advantages over other capsid-only virions, such as protection from enzymes and certain chemicals. The proteins in it can include glycoproteins functioning as receptor molecules, allowing host cells to recognize and bind these virions, resulting in the possible uptake of the virion into the cell. Not to be confused with Peptidoglycan. Glycoproteins are proteins that contain Oligosaccharide chains ( Glycans) covalently attached In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling Most enveloped viruses are dependent on the envelope for infectivity.
Complex viruses
Diagram of a bacteriophage	These viruses possess a capsid which is neither purely helical, nor purely icosahedral, and which may possess extra structures such as protein tails or a complex outer wall. Some bacteriophages have a complex structure consisting of an icosahedral head bound to a helical tail, the latter of which may have a hexagonal base plate with protruding protein tail fibres. This article is about a biological infectious particle for other uses see Phage (disambiguation.
Poxvirus	The Poxviruses are large, complex viruses which have an unusual morphology. Poxviruses (members of the family Poxviridae) are viruses that can as a family infect both Vertebrate and Invertebrate animals The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism The viral genome is associated with proteins within a central disk structure known as a nucleoid. In Prokaryotes, the nucleoid (meaning nucleus-like) is an irregularly-shaped region within the cell of Prokaryotes where the Genetic material The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface. The whole particle is slightly pleiomorphic, ranging from ovoid to brick shape. Pleomorphism is the occurrence of two or more structural forms during a life cycle, especially of certain plants [34]

Electron microscopy

For more details on this topic, see Electron microscopy. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image

The range of sizes shown by viruses, relative to those of other organisms and biomolecules

Electron microscopy is the most common method used to study the morphology of viruses. A biomolecule is any organic Molecule that is produced by living Organisms including large Polymeric molecules such as Proteins An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image The term morphology in Biology refers to the outward appearance ( Shape, Structure, Colour, Pattern) of an Organism To increase the contrast between viruses and the background, electron-dense "stains" are used. These are solutions of salts of heavy metals such as tungsten, that scatter the electrons from regions covered with the stain. Tungsten (ˈtʌŋstən also known as wolfram (/ˈwʊlfrəm/ is a Chemical element that has the symbol W and Atomic number 74 When virus particles are coated with stain (positive staining), fine detail is obscured. Negative staining overcomes this problem by staining the background only. Negative staining is an established method often used in Diagnostic microscopy, for contrasting a thin specimen with an optically opaque fluid ^[35]

Size

A medium-sized virion next to a flea is roughly equivalent to a human next to a mountain twice the size of Mount Everest. Mount Everest, also called Sagarmatha (सगरमाथा meaning Head of the Sky) or Chomolungma, Qomolangma or Zhumulangma (in Some filoviruses have a total length of up to 1400 nm, however their capsid diameters are only about 80 nm. Filoviridae is the family of Viruses that belong to the order Mononegavirales. Most viruses which have been studied have a capsid diameter between 10 and 300 nanometres. For the leaf bug see Miridae. A capsid is the protein shell of a virus. A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a Most viruses are unable to be seen with a light microscope but some are as large or larger than the smallest bacteria and can be seen under high optical magnification. The optical microscope, often referred to as the "light microscope" is a type of Microscope which uses Visible light and a system of lenses to More commonly, both scanning and transmission electron microscopes are used to visualize virus particles. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image

Genome

An enormous variety of genomic structures can be seen among viral species; as a group they contain more structural genomic diversity than the entire kingdoms of either plants, animals, or bacteria. Sense, when applied in a Molecular biology context is a general concept used to compare the polarity of Nucleic acid molecules such as DNA or RNA ^[36]

Nucleic acid

A virus may employ either DNA or RNA as the nucleic acid. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Rarely do they contain both, however cytomegalovirus is an exception to this, possessing a DNA core with several mRNA segments. Cytomegalovirus (CMV (from the Greek cyto-, "cell" and -megalo-, "large" is a viral Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product ^[18] By far most viruses have RNA. Plant viruses tend to have single-stranded RNA and bacteriophages tend to have double-stranded DNA. ^[18] Some virus species possess abnormal nucleotides, such as hydroxymethylcytosine instead of cytosine, as a normal part of their genome. Nucleotides are Organic compounds that consist of three joined structures a nitrogenous base a Sugar, and a Phosphate group Cytosine is one of the five main bases found in DNA and RNA. It is a Pyrimidine derivative with a Heterocyclic Aromatic ring ^[18]

Shape

Viral genomes may be circular, such as polyomaviruses, or linear, such as adenoviruses. Polyomavirus is the sole Genus of Viruses within the family Polyomaviridae. Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA The type of nucleic acid is irrelevant to the shape of the genome. Among RNA viruses, the genome is often divided up into separate parts within the virion and are called segmented. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Double-stranded RNA genomes and some single-stranded RNA genomes are segmented. ^[18] Each segment often codes for one protein and they are usually found together in one capsid. Every segment is not required to be in the same virion for the overall virus to be infectious, as demonstrated by the brome mosaic virus. Brome mosaic virus ( BMV) is a small (27 nm, 86S positive-stranded Icosahedral RNA plant virus belonging to the family Bromoviridae of ^[18]

Strandedness

A viral genome, irrespective of nucleic acid type, may be either single-stranded or double-stranded. Single-stranded genomes consist of an unpaired nucleic acid, analogous to one-half of a ladder split down the middle. Double-stranded genomes consist of two complementary paired nucleic acids, analogous to a ladder. Viruses, such as those belonging to the Hepadnaviridae, contain a genome which is partially double-stranded and partially single-stranded. Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals ^[36] Viruses that infect humans include double-stranded RNA (e. g. Rotavirus), single-stranded RNA (e. Rotavirus is a Genus of Double-stranded RNA virus in the family Reoviridae. g. Influenza virus), single-stranded DNA (e. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that g. Parvovirus B19) and double-stranded DNA (Herpes virus). Parvovirus B19 ( B19 virus) was the first (and until 2005 the only known human Parvovirus. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans

Sense

For viruses with RNA as their nucleic acid, the strands are said to be either positive-sense (called the plus-strand) or negative-sense (called the minus-strand), depending on whether it is complementary to viral mRNA. Sense, when applied in a Molecular biology context is a general concept used to compare the polarity of Nucleic acid molecules such as DNA or RNA Sense, when applied in a Molecular biology context is a general concept used to compare the polarity of Nucleic acid molecules such as DNA or RNA Positive-sense viral RNA is identical to viral mRNA and thus can be immediately translated by the host cell. Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. RNA polymerase ( RNAP or RNApol) is an Enzyme that produces RNA. DNA nomenclature is similar to RNA nomenclature, in that the coding strand for the viral mRNA is complementary to it (-), and the non-coding strand is a copy of it (+).

Genome size

Genome size in terms of the weight of nucleotides varies between species. Nucleotides are Organic compounds that consist of three joined structures a nitrogenous base a Sugar, and a Phosphate group The smallest genomes code for only four proteins and weigh about 10⁶ Daltons, the largest weigh about 10⁸ Daltons and code for over one hundred proteins. The unified atomic mass unit ( u) or Dalton ( Da) or sometimes universal mass unit, is an unit of Mass used to express ^[18] RNA viruses generally have smaller genome sizes than DNA viruses due to a higher error-rate when replicating, resulting in a maximum upper size limit. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. Beyond this limit, errors in the genome when replicating render the virus useless or uncompetitive. To compensate for this, RNA viruses often have segmented genomes where the genome is split into smaller molecules, thus reducing the chance of error. ^[37] In contrast, DNA viruses generally have larger genomes due to the high fidelity of their replication enzymes. ^[36]

Gene reassortment

There is an evolutionary advantage in having a segmented genome. Different strains of a virus with a segmented genome, from a pig or a bird or a human for example, such as Influenza virus, can shuffle and combine with other genes producing progeny viruses or (offspring) that have unique characteristics. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that This is called reassortment or viral sex. ^[38] This is one reason why Influenza virus constantly changes. ^[39]

Genetic recombination

Genetic recombination is the process by which a strand of DNA is broken and then joined to the end of a different DNA molecule. Genetic recombination is the process by which a strand of genetic material (usually DNA; but can also be RNA) is broken and then joined to a different DNA molecule This can occur when viruses infect cells simultaneously and studies of viral evolution have shown that recombination has been rampant in the species studied. ^[40] Recombination is common to both RNA and DNA viruses. ^[41][42]

Genetic change

Viruses undergo genetic change by several mechanisms. These include a process called genetic drift where individual bases in the DNA or RNA mutate to other bases. In Population genetics, genetic drift is the accumulation of random events that change the makeup of a gene pool slightly but often compound over time In biology mutations are changes to the Nucleotide sequence of the Genetic material of an organism Most of these point mutations are silent in that they do not change the protein that the gene encodes, but others can confer evolutionary advantages such as resistance to antiviral drugs. A point mutation, or single base substitution, is a type of Mutation that causes the replacement of a single base nucleotide with another nucleotide of the genetic Antiviral drugs are a class of Medication used specifically for treating viral Infections Like Antibiotics for Bacteria, specific antivirals ^[43] Antigenic shift is where there is a major change in the genome of the virus. Antigenic shift is the process by which at least two different strains of a virus (or different viruses especially Influenza, combine to form a new subtype In classical genetics the genome of a Diploid Organism including Eukarya refers to a full set of chromosomes or genes in a Gamete, thereby This occurs as a result of recombination or reassortment (see above). Genetic recombination is the process by which a strand of genetic material (usually DNA; but can also be RNA) is broken and then joined to a different DNA molecule Reassortment is the mixing of the genetic material of two similar Viruses that are infecting the same cell When this happens with influenza viruses, pandemics may result. A pandemic (from Greek παν pan all + δήμος demos people is an Epidemic of Infectious disease that spreads through ^[44][45] By genome rearrangement the structure of the gene changes although no mutations have necessarily occurred. ^[46]

RNA viruses are much more likely to mutate than DNA viruses for the reasons outlined above. Viruses often exist as quasispecies or swarms of viruses of the same species but with slightly different genome nucleoside sequences. The quasispecies model is a description of the process of the Darwinian Evolution of certain self-replicating entities within the framework of physical chemistry Such quasispecies are a prime target for natural selection. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of ^[47]

Replication

Viral populations do not grow through cell division, because they are acellular; instead, they use the machinery and metabolism of a host cell to produce multiple copies of themselves. Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. A virus can still cause degenerative effects within a cell without causing its death; collectively these are termed cytopathic effects. Cytopathic effect (CPE refers to degenerative changes in cells (especially in Tissue culture) associated with the multiplication of certain Viruses

Virus life cycle

The life cycle of viruses differs greatly between species (see below) but there are six basic stages in the life cycle of viruses:

A virus attaches to the host cell and enters endocytosis. Chronic since 1992 ( talk) 2200 12 December 2007 (UTC --> Viruses are The capsid protein dissociates and the viral RNA is transported to the nucleus. In the nucleus, the viral polymerase complexes transcribe and replicate the RNA. Viral mRNAs migrate to cytoplasm where they are translated into protein. Then the newly synthesized virions bud from infected cell. Chronic since 1992 ( talk) 2200 12 December 2007 (UTC --> Viral shedding

• Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, the human immunodeficiency virus (HIV) infects only human T cells, because its surface protein, gp120, can interact with CD4 and receptors on the T cell's surface. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. gp120 is a Glycoprotein exposed on the surface of the HIV envelope. CD4 ( Cluster of differentiation 4 is a Glycoprotein expressed on the surface of T helper cells, Regulatory T cells, Monocytes, This mechanism has evolved to favour those viruses that only infect cells that they are capable of replicating in. Attachment to the receptor can induce the viral-envelope protein to undergo changes that results in the fusion of viral and cellular membranes.
• Penetration: following attachment, viruses enter the host cell through receptor mediated endocytosis or membrane fusion. Endocytosis is a process where cells absorb material ( Molecules such as proteins from the outside by engulfing it with their Cell membrane.
• Uncoating is a process in which the viral capsid is degraded by viral enzymes or host enzymes thus releasing the viral genomic nucleic acid. For the leaf bug see Miridae. A capsid is the protein shell of a virus. Enzymes are Biomolecules that catalyze ( ie increase the rates of Chemical reactions Almost all enzymes are Proteins
• Replication involves synthesis of viral messenger RNA (mRNA) for viruses except positive sense RNA viruses (see above), viral protein synthesis and assembly of viral proteins and viral genome replication. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Protein biosynthesis (synthesis is the process in which cells build Proteins The term is sometimes used to refer only to protein translation but more
• Following the assembly of the virus particles post-translational modification of the viral proteins often occurs. In viruses such as HIV, this modification, (sometimes called maturation), occurs after the virus has been released from the host cell. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome ^[48]
• Viruses are released from the host cell by lysis (see below). Enveloped viruses (e. g. , HIV) typically are released from the host cell by budding. Chronic since 1992 ( talk) 2200 12 December 2007 (UTC --> Viral shedding During this process, the virus acquires its phospholipid envelope which contains embedded viral glycoproteins.

DNA viruses

Animal DNA viruses, such as herpesviruses, enter the host via endocytosis, the process by which cells take in material from the external environment. A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Endocytosis is a process where cells absorb material ( Molecules such as proteins from the outside by engulfing it with their Cell membrane. Frequently after a chance collision with an appropriate surface receptor on a cell, the virus penetrates the cell, the viral genome is released from the capsid, and host polymerases begin transcribing viral mRNA. New virions are assembled and released either by cell lysis or by budding off the cell membrane. Chronic since 1992 ( talk) 2200 12 December 2007 (UTC --> Viral shedding

RNA viruses

Animal RNA viruses can be placed into about four different groups depending on their modes of replication. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. The polarity of the RNA largely determines the replicative mechanism, as well as whether the genetic material is single-stranded or double-stranded. Sense, when applied in a Molecular biology context is a general concept used to compare the polarity of Nucleic acid molecules such as DNA or RNA Some RNA viruses are actually DNA-based but use an RNA-intermediate to replicate. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. RNA viruses are dependent on virally encoded RNA replicase to create copies of their genomes. RNA-dependent RNA polymerase ( RDRP) or RNA replicase, is an Enzyme that catalyzes the replication of RNA from an RNA template

Reverse transcribing viruses

Reverse transcribing viruses replicate using reverse transcription, which is the formation of DNA from an RNA template. A reverse transcribing virus is any virus which replicates using Reverse transcription, the formation of DNA from an RNA template Reverse transcribing viruses containing RNA genomes use a DNA intermediate to replicate, whereas those containing DNA genomes use an RNA intermediate during genome replication. Both types use the reverse transcriptase enzyme to carry out the nucleic acid conversion. In Biochemistry, a reverse transcriptase, also known as RNA-dependent DNA polymerase, is a DNA polymerase Enzyme that transcribes Both types are susceptible to antiviral drugs that inhibit the reverse transcriptase enzyme, e. Antiviral drugs are a class of Medication used specifically for treating viral Infections Like Antibiotics for Bacteria, specific antivirals g. zidovudine and lamivudine. Zidovudine ( INN) or azidothymidine ( AZT) (also called ZDV is a nucleoside analog reverse transcriptase inhibitor (NRTI a type of Antiretroviral Lamivudine (2'3'-dideoxy-3'-thia Cytidine, commonly

An example of the first type is HIV which is a retrovirus. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome A retrovirus is any Virus belonging to the viral family Retroviridae. Retroviruses often integrate the DNA produced by reverse transcription into the host genome. Reverse transcription is the process of making a double stranded DNA (deoxyribonucleic acid molecule from a single stranded RNA (ribonucleic acid template This is why HIV infection can at present, only be treated and not cured.

Examples of the second type are the Hepadnaviridae, which includes the Hepatitis B virus and the Caulimoviridae - e. Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals General Overview The Caulimoviridae are a family of Viruses including the following genera Genus Badnavirus; type g. Cauliflower mosaic virus. Cauliflower mosaic virus ( CaMV) is the type member of the Caulimoviruses one of the six genera in the Caulimoviridae family Pararetroviruses

Bacteriophages

Main article: Bacteriophage

Transmission electron micrograph of multiple bacteriophages attached to a bacterial cell wall

Bacteriophages infect specific bacteria by binding to surface receptor molecules and then enter the cell. This article is about a biological infectious particle for other uses see Phage (disambiguation. This article is about a biological infectious particle for other uses see Phage (disambiguation. This article is about a biological infectious particle for other uses see Phage (disambiguation. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling Within a short amount of time, in some cases, just minutes, bacterial polymerase starts translating viral mRNA into protein. A polymerase (EC 2776/7/19/48/49 is an Enzyme whose central function is associated with Polymers of Nucleic acids such as RNA and DNA These proteins go on to become either new virions within the cell, helper proteins which help assembly of new virions, or proteins involved in cell lysis. Lysis ( Greek, lysis from lyein = to separate refers to the death of a cell by breaking of the cellular membrane often by viral or osmotic Viral enzymes aid in the breakdown of the cell membrane, and in the case of the T4 phage, in just over twenty minutes after injection over three hundred phages could be released. Enterobacteria phage T4 is a Phage that infects E coli bacteria.

Lifeform debate

Viruses have been described as "organisms at the edge of life",^[49] but argument continues over whether viruses are truly alive. According to the United States Code, they are considered microorganisms in the sense of biological weaponry and malicious use. The United States Code ( USC) is a compilation and Codification of the general and permanent federal Law of the United States. A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually Biological warfare (BW — known as a germ warfare, biological weapons and bioweaponry — is the use of any Pathogen ( Bacterium Scientists, however, are divided. Things become more complicated as they look at viroids and prions. Viroids are Plant Pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary circular single-stranded A prion (ˈpriːɒn is thought to be an infectious agent that according to current scientific consensus is comprised entirely of a propagated, mis-folded Viruses resemble other organisms in that they possess genes and can evolve in infected cells by natural selection. Natural selection is the process by which favorable Heritable traits become more common in successive Generations of a Population of ^[50][51] They can reproduce by creating multiple copies of themselves through self-assembly.

Viruses do not have a cell structure (regarded as the basic unit of life), although they do have genes. The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called Additionally, although they reproduce, they do not self-metabolize and require a host cell to replicate and synthesize new products. However, bacterial species such as Rickettsia and Chlamydia are considered living organisms but are unable to reproduce outside a host cell. Rickettsia is a Genus of motile, Gram-negative, non-sporeforming, highly Pleomorphic bacteria that can present

An argument can be made that accepted forms of life use cell division to reproduce, whereas viruses spontaneously assemble within cells. Cell division is a process by which a cell, called the parent cell divides into two or more cells called daughter cells. The comparison is drawn between viral self-assembly and the autonomous growth of non-living crystals. Crystallization is the (natural or artificial process of formation of solid Crystals precipitating from a homogeneous --> identical Solution Virus self-assembly within host cells has implications for the study of the origin of life, as it lends credence to the hypothesis that life could have started as self-assembling organic molecules. In the Natural sciences, Abiogenesis, or origin of life, is the study of how Life on Earth emerged from Inanimate Organic ^[52]

If viruses are considered alive, then the criteria specifying life will have to exclude the cell. If viruses are said to be alive, the question could follow of whether even smaller infectious particles, such as viroids and prions, are alive. Viroids are Plant Pathogens that consist of a short stretch (a few hundred nucleobases) of highly complementary circular single-stranded A prion (ˈpriːɒn is thought to be an infectious agent that according to current scientific consensus is comprised entirely of a propagated, mis-folded

Viruses and disease

For more examples of diseases caused by viruses see List of infectious diseases. Human Infectious diseases grouped by causative agent and alphabetically arranged

Examples of common human diseases caused by viruses include the common cold, the flu, chickenpox and cold sores. Acute viral nasopharyngitis or acute coryza, usually known as the common cold, is a highly contagious viral Infectious disease of the Chickenpox is a highly contagious illness caused by primary infection with Varicella zoster virus (VZV Herpes simplex is a Viral disease caused by Herpes simplex viruses both herpes simplex virus 1 (HSV-1 and herpes simplex virus 2 (HSV-2 cause herpes simplex Serious diseases such as Ebola, AIDS, avian influenza and SARS are caused by viruses. Ebola is the common term for a group of Viruses belonging to genus Ebolavirus, family Filoviridae, and for the disease which they For the H5N1 subtype of Avian influenza see H5N1. Avian influenza, sometimes Avian flu, and commonly Bird flu refers The relative ability of viruses to cause disease is described in terms of virulence. Virulence (also called pestiferousness) refers to the degree of Pathogenicity of a Microbe, or in other words the relative ability of a Microbe Other diseases are under investigation as to whether they too have a virus as the causative agent, such as the possible connection between Human Herpesvirus Six (HHV6) and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the Chronic fatigue syndrome ( CFS) is the most common name given to a poorly understood variably debilitating disorder or disorders of uncertain causation There is current controversy over whether the borna virus, previously thought of as causing neurological diseases in horses, could be responsible for psychiatric illnesses in humans. BDV redirects here for Big Daddy V see Nelson Frazier Jr Borna disease is an infectious Neurological syndrome of Psychiatry is a medical specialty which exists to study, prevent, and treat Mental disorders in Humans Psychiatric ^[53]

Viruses have different mechanisms by which they produce disease in an organism, which largely depends on the species. Mechanisms at the cellular level primarily include cell lysis, the breaking open and subsequent death of the cell. Lysis ( Greek, lysis from lyein = to separate refers to the death of a cell by breaking of the cellular membrane often by viral or osmotic In multicellular organisms, if enough cells die the whole organism will start to suffer the effects. Multicellular organisms are Organisms consisting of more than one cell, and having Differentiated cells that perform specialized functions Although viruses cause disruption of healthy homeostasis, resulting in disease, they may exist relatively harmlessly within an organism. Homeostasis (from Greek: ὅμος hómos, "equal" and ιστημι istēmi, "to stand" lit An example would include the ability of the herpes simplex virus, which cause cold sores, to remain in a dormant state within the human body. Herpes simplex virus 1 and 2 ( HSV-1 and HSV-2) are two species of the herpes virus family Herpesviridae, which cause infections This is called latency^[54] and is a characteristic of the herpes viruses including the Epstein-Barr virus, which causes glandular fever, and the Varicella zoster virus, which causes chicken pox. The Epstein-Barr Virus ( EBV) also called Human herpesvirus 4 (HHV-4 is a Virus of the herpes family (which includes Herpes Varicella zoster virus ( VZV) is one of eight herpes viruses known to infect humans (and other vertebrates Chickenpox is a highly contagious illness caused by primary infection with Varicella zoster virus (VZV Latent chickenpox infections return in later life as the disease called shingles. Herpes zoster (or simply zoster) commonly known as shingles, is a Viral disease characterized by a painful skin rash with Blisters in a limited

Some viruses can cause life-long or chronic infections, where the viruses continue to replicate in the body despite the hosts' defense mechanisms. In Medicine, a chronic disease is a Disease that is long-lasting or recurrent ^[55] This is common in Hepatitis B virus and Hepatitis C Virus infections. This page is for the virus For the disease see Hepatitis C. Hepatitis C virus ( HCV) is a small (50 nm in size People chronically infected with the Hepatitis B virus are known as carriers who serve as reservoirs of infectious virus. In some populations, with a high proportion of carriers, the disease is said to be endemic. In Epidemiology, an Infection is said to be endemic (from Greek en- in or within + demos people in a Population when ^[56] When diagnosing Hepatitis B virus infections, it is important to distinguish between acute and chronic infections. In Medicine, an acute disease is a disease with either or both of a rapid onset a short course (as opposed to a chronic course In Medicine, a chronic disease is a Disease that is long-lasting or recurrent ^[57]

Epidemiology

Viral epidemiology is the branch of medical science dealing with the transmission and control of virus infections in humans. Epidemiology is the study of factors affecting the Health and Illness of populations and serves as the foundation and Logic of interventions made in the Transmission of viruses can be vertical, that is from mother to child, or horizontal, which means from person to person. Examples of vertical transmission include Hepatitis B virus and HIV where the baby is born already infected with the virus. Vertical transmission, also known as Mother-to-child transmission refers to transmission of an Infection, such as HIV, Hepatitis B, or Hepatitis Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome ^[58] Another, more rare, example is the Varicella zoster virus, which although causing relatively mild infections in humans, can be fatal to the foetus and newly born baby. Varicella zoster virus ( VZV) is one of eight herpes viruses known to infect humans (and other vertebrates ^[59] Horizontal transmission is the most common mechanism of spread of viruses in populations. Horizontal transmission is the transmission of a Bacterial, Fungal, or Viral Infection between members of the same species that are not in a Transmission can be exchange of blood by sexual activity, e. g. HIV, Hepatitis B and Hepatitis C; by mouth by exchange of saliva, e. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome Hepatitis C is a blood-borne infectious disease that is caused by the Hepatitis C virus ( HCV) affecting the Liver. For the band see Saliva (band; for the village in Azerbaijan see Səliva. g. Epstein-Barr virus, or from contaminated food or water, e. The Epstein-Barr Virus ( EBV) also called Human herpesvirus 4 (HHV-4 is a Virus of the herpes family (which includes Herpes g. Norovirus; by breathing in viruses in the form of aerosols, e. Norovirus (was "Norwalk-like viruses" an RNA virus of the Caliciviridae taxonomic family, causes approximately 90% of Epidemic Technically an aerosol is a suspension of fine solid particles or liquid droplets in a gas g. Influenza virus; and by insect vectors such as mosquitoes, e. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that g. dengue. The rate or speed of transmission of viral infections depends on factors that include population density, the number of susceptible individuals, (i. Population density (in agriculture standing stock and Standing crop) is a measurement of Population per unit area or unit volume e. those who are not immune),^[60] the quality of health care and the weather. ^[61]

Epidemics and pandemics

For more details on this topic, see List of epidemics. This article is a list of major Epidemics. Worldwide pandemics The following are Epidemics which spread across several continents

The reconstructed 1918 influenza virus

Native American populations were devastated by contagious diseases, particularly smallpox, brought to the Americas by European colonists. The 1918 flu pandemic (commonly referred to as the Spanish flu) was an Influenza Pandemic that spread to nearly every part of the world For indigenous peoples in the United States other than Hawaii and Alaska see also Native Americans in the United States. Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor. It is unclear how many Native Americans were killed by foreign diseases after the arrival of Columbus in the Americas, but the numbers have been estimated to be close to 70% of the indigenous population. The damage done by this disease significantly aided European attempts to displace and conquer the native population. ^{[62][63][64][65][66][67][68]}

Main article: Spanish flu

A pandemic is a world-wide epidemic. The 1918 flu pandemic (commonly referred to as the Spanish flu) was an Influenza Pandemic that spread to nearly every part of the world A pandemic (from Greek παν pan all + δήμος demos people is an Epidemic of Infectious disease that spreads through The 1918 flu pandemic, commonly referred to as the Spanish flu, was a category 5 influenza pandemic caused by an unusually severe and deadly Influenza A virus. The 1918 flu pandemic (commonly referred to as the Spanish flu) was an Influenza Pandemic that spread to nearly every part of the world The Pandemic Severity Index (PSI is a proposed classification scale for reporting the severity of Influenza pandemics in the United States. Influenzavirus A is a Genus of the Orthomyxoviridae family of Viruses Influenzavirus A includes only one Species: Influenza A virus The victims were often healthy young adults, in contrast to most influenza outbreaks which predominantly affect juvenile, elderly, or otherwise weakened patients.
The Spanish flu pandemic lasted from 1918 to 1919. The 1918 flu pandemic (commonly referred to as the Spanish flu) was an Influenza Pandemic that spread to nearly every part of the world Older estimates say it killed 40–50 million people,^[69] while more recent research suggests that it may have killed as many as 100 million people, or 5% of the world's population in 1918. ^[70]

Main article: AIDS

The Ebola virus

Most researchers believe that HIV originated in sub-Saharan Africa during the twentieth century;^[71] it is now a pandemic, with an estimated 38. Ebola is the common term for a group of Viruses belonging to genus Ebolavirus, family Filoviridae, and for the disease which they Sub-Saharan Africa is a geographical term used to describe the area of the African continent which lies south of the Sahara, or those African countries The twentieth century of the Common Era began on A pandemic (from Greek παν pan all + δήμος demos people is an Epidemic of Infectious disease that spreads through 6 million people now living with the disease worldwide. The word million In standard English, the -lli- in million is pronounced with an l-sound followed by a ^[72] As of January 2006, the Joint United Nations Programme on HIV/AIDS (UNAIDS) and the World Health Organization (WHO) estimate that AIDS has killed more than 25 million people since it was first recognized on June 5, 1981, making it one of the most destructive epidemics in recorded history. The Joint United Nations Programme on HIV and AIDS, or UNAIDS, is the main advocate for accelerated comprehensive and coordinated global action Events 70 - Titus and his Roman Legions breach the middle wall of Jerusalem in the Siege of Jerusalem Year 1981 ( MCMLXXXI) was a Common year starting on Thursday (link displays the 1981 In Epidemiology, an epidemic (from Greek epi- upon + demos people is a classification of a disease that appears as new cases in a Recorded history can be defined as History that has been written down or recorded by the use of Language, whereas History is a more general term referring ^[73]

The Marburg virus

Main article: Ebola

Several highly lethal viral pathogens are members of the Filoviridae. The Marburg virus is the causative agent of Marburg haemorrhagic fever (also known as green monkey disease) Ebola is the common term for a group of Viruses belonging to genus Ebolavirus, family Filoviridae, and for the disease which they Filoviridae is the family of Viruses that belong to the order Mononegavirales. Filoviruses are filament-like viruses that cause viral hemorrhagic fever, and include the Ebola and Marburg viruses. The viral hemorrhagic Fevers ( VHFs') are a diverse group of animal and human illnesses that are caused by five distinct families of Ebola is the common term for a group of Viruses belonging to genus Ebolavirus, family Filoviridae, and for the disease which they The Marburg virus is the causative agent of Marburg haemorrhagic fever (also known as green monkey disease) The Marburg virus attracted widespread press attention in April 2005 for an outbreak in Angola. Angola, officially the Republic of Angola (República de Angola Pronounced ʁɛˈpublikɐ dɨ ɐ̃ˈgɔlɐ Repubilika ya Ngola is a country in south-central Beginning in October 2004 and continuing into 2005, the outbreak was the world's worst epidemic of any kind of viral hemorrhagic fever. ^[74]

Viruses and cancer

For more details on this topic, see Oncovirus. An oncovirus is a Virus associated with Cancer. Oncoviruses come in two different forms viruses with a DNA Genome, such as Adenovirus

Human leukaemia cells infected by the Epstein Barr virus

Viruses are an established cause of malignancy in humans and other species. The Epstein-Barr Virus ( EBV) also called Human herpesvirus 4 (HHV-4 is a Virus of the herpes family (which includes Herpes Cancer (medical term Malignant Neoplasm) is a class of Diseases in which a group of cells display uncontrolled The main viruses associated with human cancers are human papillomavirus, hepatitis B and hepatitis C virus, Epstein-Barr virus, and human T-lymphotropic virus. A human papillomavirus ( HPV) is a Papillomavirus that infects the skin and Mucous membranes of Humans Approximately 130 HPV types have been Hepatitis C is a blood-borne infectious disease that is caused by the Hepatitis C virus ( HCV) affecting the Liver. The Epstein-Barr Virus ( EBV) also called Human herpesvirus 4 (HHV-4 is a Virus of the herpes family (which includes Herpes Human T-lymphotropic virus (HTLV is a Human, single-stranded RNA Retrovirus that causes T-cell Leukemia and T-cell Lymphoma Hepatitis viruses, including hepatitis B and hepatitis C, can induce a chronic viral infection that leads to liver cancer. Hepatitis C is a blood-borne infectious disease that is caused by the Hepatitis C virus ( HCV) affecting the Liver. In Medicine, a chronic disease is a Disease that is long-lasting or recurrent Hepatocellular carcinoma (HCC also called hepatoma) is a primary malignancy (cancer of the Liver. ^[75][76] Infection by human T-lymphotropic virus can lead to tropical spastic paraparesis and adult T-cell leukemia. Human T-lymphotropic virus (HTLV is a Human, single-stranded RNA Retrovirus that causes T-cell Leukemia and T-cell Lymphoma Tropical spastic paraparesis ( TSP) is an Infection of the Spinal cord by Human T-lymphotropic virus resulting in Paraparesis or weakness Adult T cell leukemia ( ATL) is a rare Cancer of the immune system's own T-cells Human T cell leukemia/lymphotropic virus type 1 ( HTLV-1) is believed ^[77] Human papillomaviruses are an established cause of cancers of cervix, skin, anus, and penis. A human papillomavirus ( HPV) is a Papillomavirus that infects the skin and Mucous membranes of Humans Approximately 130 HPV types have been The cervix (from Latin "neck" is the lower narrow portion of the Uterus where it joins with the top end of the Vagina. The anus is an opening at the opposite end of an Animal 's Digestive tract from the Mouth. The penis (plural penises, penes ^[78] Within the Herpesviridae, Kaposi's sarcoma-associated herpesvirus causes Kaposi's sarcoma and body cavity lymphoma, and Epstein–Barr virus causes Burkitt's lymphoma, Hodgkin’s lymphoma, B lymphoproliferative disorder and nasopharyngeal carcinoma. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Kaposi's sarcoma-associated herpesvirus ( KSHV) is the eighth human Herpesvirus; its formal name according to the International Committee on Taxonomy of Viruses is Kaposi's sarcoma (KS is a tumor caused by Human herpesvirus 8 ( HHV8) also known as Kaposi's sarcoma-associated herpesvirus (KSHV The Epstein-Barr Virus ( EBV) also called Human herpesvirus 4 (HHV-4 is a Virus of the herpes family (which includes Herpes Burkitt lymphoma (or "Burkitt's tumor" or "Malignant lymphoma Burkitt's type" is a cancer of the lymphatic system (in particular B lymphocytes Hodgkin's lymphoma, also known as Hodgkin's disease is a type of Lymphoma first described by Thomas Hodgkin in 1832 B cells are Lymphocytes that play a large role in the humoral immune response (as opposed to the cell-mediated immune response, which is governed by Lymphoproliferative disorders (LPDs refer to several conditions in which Lymphocytes are produced in excessive quantities Nasopharyngeal carcinoma (NPC is a Cancer originating in the Nasopharynx, the uppermost region of the Pharynx or "throat" where the nasal ^[79]

Laboratory diagnosis

Cells infected with Herpes simplex virus. Herpes simplex virus 1 and 2 ( HSV-1 and HSV-2) are two species of the herpes virus family Herpesviridae, which cause infections The rounding of the cells, their detachment from the cell sheet is the typical cytopathic effect produced by this virus.

Main article: Laboratory diagnosis of virus

In the diagnostic laboratory, virus infections are confirmed by several methods that include:

• Growth of the virus in a cell culture from a specimen taken from the patient. Cell culture is the process by which prokaryotic, or eukaryotic cells are grown under controlled conditions
• Detection of virus-specific IgM antibody (see below) in the blood. Immunoglobulin M, or IgM for short is a basic Antibody that is present on B cells It is the primary antibody against A and B
• Detection of virus antigens by ELISA in tissues and fluids. Enzyme-Linked ImmunoSorbent Assay, also called ELISA, Enzyme ImmunoAssay or EIA, is a biochemical technique used mainly in Immunology
• Detection of virus encoded DNA and RNA by PCR.
• Observation of virus particles by electron microscopy. An electron microscope is a type of Microscope that uses Electrons to illuminate a specimen and create an enlarged image

Prevention and treatment

Because viruses use the machinery of a host cell to reproduce and reside within them, they are difficult to eliminate without killing the host cell. The most effective medical approaches to viral diseases so far are vaccinations to provide resistance to infection, and antiviral drugs which treat the symptoms of viral infections. Medicine is the art and science of healing It encompasses a range of Health care practices evolved to maintain and restore Human Health by the Vaccination is the administration of Antigenic material (the Vaccine) to produce immunity to a disease Antiviral drugs are a class of Medication used specifically for treating viral Infections Like Antibiotics for Bacteria, specific antivirals

Host immune response

The body's first line of defense against viruses is the innate immune system. Immune system|Adaptive immune systemThe innate immune system comprises the cells and mechanisms that defend the host from infection by other organisms in a non-specific manner This comprises cells and other mechanisms that defend the host from infection in a non-specific manner. This means that the cells of the innate system recognize, and respond to, pathogens in a generic way, but unlike the adaptive immune system, it does not confer long-lasting or protective immunity to the host. Immune system|Passive immunity|Innate immune system The adaptive immune system is composed of highly specialized systemic cells and processes that eliminate or prevent Pathogenic ^[80]

RNA interference is an important innate defense against viruses. RNA interference ( RNAi) is a mechanism that inhibits Gene expression at the stage of translation or by hindering the transcription of specific ^[81] Many viruses have a replication strategy that involves double-stranded RNA dsRNA. Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units When such a virus infects a cell, it releases its RNA molecule or molecules, which immediately bind to a protein complex called Dicer that cuts the RNA into smaller pieces. Dicer is a Ribonuclease in the RNase III family that cleaves double-stranded RNA (dsRNA and pre- MicroRNA (miRNA into short double-stranded A biochemical pathway called the RISC complex is activated which degrades the viral mRNA and the cell survives the infection. Messenger ribonucleic acid ( mRNA) is a molecule of RNA encoding a chemical "blueprint" for a Protein product Rotaviruses avoid this mechanism by not uncoating fully inside the cell and by releasing newly produced mRNA through pores in the particle's inner capsid. The genomic dsRNA remains protected inside the core of the virion. Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units ^[82][83]

When the adaptive immune system of a vertebrate encounters a virus, it produces specific antibodies which bind to the virus and render it non-infectious. Immune system|Passive immunity|Innate immune system The adaptive immune system is composed of highly specialized systemic cells and processes that eliminate or prevent Pathogenic Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Antibodies (also known as immunoglobulins, abbreviated Ig) are Gamma globulin Proteins that are found in Blood or other Bodily This is called humoral immunity. The Humoral Immune Response (HIR is the aspect of immunity that is mediated by secreted Antibodies (as opposed to Cell-mediated immunity which involves Two types of antibodies are important. The first called IgM is highly effective at neutralizing viruses but is only produced by the cells of the immune system for a few weeks. Immunoglobulin M, or IgM for short is a basic Antibody that is present on B cells It is the primary antibody against A and B The second, called, IgG is produced indefinitely. Immunoglobulin G ( IgG) is a Monomeric Immunoglobulin, built of two heavy chains γ and two light chains. The presence of IgM in the blood of the host is used to test for acute infection, whereas IgG indicates an infection sometime in the past. ^[84] Both types of antibodies are measured when tests for immunity are carried out. Immunity is a material term that describes a state of having sufficient biological defenses to avoid Infection, Disease, or other unwanted biological invasion ^[85]

A second defense of vertebrates against viruses is called cell-mediated immunity and involves immune cells known as T cells. Cell-mediated immunity is an Immune response that does not involve Antibodies or complement but rather involves the activation of Macrophages T cells belong to a group of White blood cells known as Lymphocytes, and play a central role in Cell-mediated immunity. The body's cells constantly display short fragments of their proteins on the cell's surface, and if a T cell recognizes a suspicious viral fragment there, the host cell is destroyed by T killer cells and the virus-specific T-cells proliferate. Cells such as the macrophage are specialists at this antigen presentation. Macrophages ( Greek: "big eaters" from makros "large" + phagein "eat" ( Mø) are cells within the tissues that Antigen presentation is a process in the body's Immune system by which Macrophages, Dendritic cells and other cell types capture Antigens and ^[86][87]

Not all virus infections produce a protective immune response in this way. HIV evades the immune system by constantly changing the amino acid sequence of the proteins on the surface of the virion. These persistent viruses evade immune control by sequestration, blockade of antigen presentation, cytokine resistance, evasion of natural killer cell activities, escape from apoptosis, and antigenic shift. Antigen presentation is a process in the body's Immune system by which Macrophages, Dendritic cells and other cell types capture Antigens and Cytokines are a category of signalling Proteins and Glycoproteins that like Hormones and Neurotransmitters, are used extensively in cellular Natural killer cells (or NK cells) are a type of Cytotoxic Lymphocyte that constitute a major component of the Innate immune system. Antigenic shift is the process by which at least two different strains of a virus (or different viruses especially Influenza, combine to form a new subtype ^[88] Other viruses, called "neurotropic viruses", are disseminated by neural spread where the immune system may be unable to reach them. A neurotropic virus is a Virus which is capable of infecting Nerve cells, or which does so preferentially An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor

The production of interferon is an important host defense mechanism. Interferons ( IFN s are natural Proteins produced by the cells of the Immune system of most Vertebrates in response to challenges by foreign agents ^[89]

Vaccines

For more details on this topic, see Vaccination. Vaccination is the administration of Antigenic material (the Vaccine) to produce immunity to a disease

Vaccination is a cheap and effective way of preventing infections by viruses. Vaccination is the administration of Antigenic material (the Vaccine) to produce immunity to a disease Vaccines were used to prevent viral infections long before the discovery of the actual viruses. Their use has resulted in a dramatic decline in morbidity (illness) and mortality (death) associated with viral infections such as polio, measles, mumps and rubella. Poliomyelitis, often called polio or infantile paralysis, is an acute viral Infectious disease spread from person to person primarily via Measles (rubeola is a Disease caused by a virus specifically a Paramyxovirus of the genus Morbillivirus. Mumps or epidemic Parotitis is a Viral disease of the Human species Rubella, commonly known as German measles, is a Disease caused by Rubella virus ^[90] Smallpox infections have been eradicated. Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor. ^[91] Currently vaccines are available to prevent over thirteen viral infections of humans,^[92] and more are used to prevent viral infections of animals. ^[93] Vaccines can consist of live-attenuated or killed viruses, or viral proteins (antigens). An antigen (from antibody-generating) or immunogen is a substance that prompts the generation of Antibodies and can cause an immune response ^[94] Live vaccines contain weakened forms of the virus that causes the disease. Such viruses are called attenuated. Live vaccines can be dangerous when given to people with a weak immunity, (who are described as immunocompromised), because in these people, the weakened virus can cause the original disease. In Medicine, immunodeficiency (or immune deficiency) is a state in which the Immune system 's ability to fight Infectious disease is compromised ^[95] Biotechnology and genetic engineering techniques are used to produce subunit vaccines. These vaccines use only the capsid proteins of the virus. For the leaf bug see Miridae. A capsid is the protein shell of a virus. Hepatitis B vaccine is an example of this type of vaccine. ^[96] Subunit vaccines are safe for immunocompromised patients because they cannot cause the disease. In Medicine, immunodeficiency (or immune deficiency) is a state in which the Immune system 's ability to fight Infectious disease is compromised ^[97] The Yellow Fever virus vaccine, a live-attenuated strain called 17D, is arguably the safest and most effective vaccine ever generated.

Antiviral drugs

For more details on this topic, see Antiviral drug. Antiviral drugs are a class of Medication used specifically for treating viral Infections Like Antibiotics for Bacteria, specific antivirals

The true DNA base thymidine

The antiviral drug Zidovudine - AZT

Over the past twenty years, the development of antiviral drugs has increased rapidly. Thymidine (more precisely called deoxythymidine; can also be labelled deoxyribosylthymine, and thymine deoxyriboside) is a Chemical compound Zidovudine ( INN) or azidothymidine ( AZT) (also called ZDV is a nucleoside analog reverse transcriptase inhibitor (NRTI a type of Antiretroviral Zidovudine ( INN) or azidothymidine ( AZT) (also called ZDV is a nucleoside analog reverse transcriptase inhibitor (NRTI a type of Antiretroviral Antiviral drugs are a class of Medication used specifically for treating viral Infections Like Antibiotics for Bacteria, specific antivirals This has been driven by the AIDS epidemic. Antiviral drugs are often nucleoside analogues, (fake DNA building blocks), which viruses incorporate into their genomes during replication. Nucleoside analogues are a range of antiviral products used to prevent Viral replication in infected cells The life-cycle of the virus is then halted because the newly synthesized DNA is inactive. This is because these analogues lack the hydroxyl groups which along with phosphorus atoms, link together to form the strong "backbone" of the DNA molecule. In Chemistry, hydroxide is the most common name for the diatomic Anion OH− consisting of Oxygen and Hydrogen Phosphorus, (ˈfɒsfərəs is the Chemical element that has the symbol P and Atomic number 15 This is called DNA chain termination. Chain termination is any Chemical reaction leading to the destruction of a Reactive intermediate in a Chain propagation step in the course of a ^[98] Examples of nucleoside analogues are aciclovir for Herpes virus infections and lamivudine for HIV and Hepatitis B virus infections. Aciclovir ( INN The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Lamivudine (2'3'-dideoxy-3'-thia Cytidine, commonly Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome Aciclovir is one of the oldest and most frequently prescribed antiviral drugs. Aciclovir ( INN ^[99]

Guanosine

The guanosine analogue Aciclovir

Other antiviral drugs in use target different stages of the viral life cycle. Guanosine is a Nucleoside comprising Guanine attached to a Ribose ( Ribofuranose) ring via a β-N9- Glycosidic bond. Aciclovir ( INN HIV is dependent on a proteolytic enzyme called the HIV-1 protease for it to become fully infectious. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome HIV-1 protease (HIV PR is an Aspartic protease that is essential for the life-cycle of HIV, the Retrovirus that causes AIDS. There is a class of drugs called protease inhibitors which have been designed to inactivate the enzyme. For natural protease inhibitors please see Protease inhibitor (biology Protease inhibitors (PIs are a class of Medications used

Hepatitis C is caused by an RNA virus. Hepatitis C is a blood-borne infectious disease that is caused by the Hepatitis C virus ( HCV) affecting the Liver. Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units In 80% of people infected, the disease is chronic, and without treatment, they are infected and infectious for the remainder of their lives. In Medicine, a chronic disease is a Disease that is long-lasting or recurrent However, there is now an effective treatment using the nucleoside analogue drug ribavirin combined with interferon. Ribavirin (Copegus Rebetol Ribasphere VilonaVirazole also generics from Sandoz Teva Warrick is an Anti-viral drug which is active against a number of DNA and Interferons ( IFN s are natural Proteins produced by the cells of the Immune system of most Vertebrates in response to challenges by foreign agents ^[100] The treatment of chronic carriers of the Hepatitis B virus by using a similar strategy using lamivudine is being developed. An asymptomatic carrier ( healthy carrier or just carrier) is a person or other organism that has contracted an Infectious disease, but who displays no symptoms Lamivudine (2'3'-dideoxy-3'-thia Cytidine, commonly ^[101]

Notable examples

Further information: Table of clinically important viruses

The clinically most notable^[102] virus species belong to the following families:

Applications

Life sciences and medicine

Viruses are important to the study of molecular and cellular biology as they provide simple systems that can be used to manipulate and investigate the functions of cells. Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA A Picornavirus is a Virus belonging to the family Picornaviridae. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals The Flaviviridae are a family of Viruses that are primarily spread through arthropod vectors (mainly ticks and mosquitoes A retrovirus is any Virus belonging to the viral family Retroviridae. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that Paramyxoviruses are Viruses of the Paramyxoviridae family of the Mononegavirales order they are negative-sense single-stranded A Papovaviridae is a member of the Papovaviridae family of Viruses Papovaviridae includes two genera: Papillomavirus and Rhabdoviruses are Viruses belonging to the family Rhabdoviridae, which is in the order Mononegavirales. Reoviridae is a family of Viruses that can affect the Gastrointestinal system (such as Rotavirus) and Respiratory tract The Togaviridae are a family of Viruses including the following genera Genus Alphavirus; type species Sindbis virus The Baltimore classification is a Virus classification system which groups Viruses into families depending on their type of Genome ( DNA Many Viruses (eg influenza and many animal viruses have viral envelopes covering their protein Capsids The envelopes are typically derived from portions of Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. A Picornavirus is a Virus belonging to the family Picornaviridae. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Coxsackievirus is the name of two groups of Enteroviruses Coxsackie A viruses * Coxsackie B viruses Coxsackieviruses belong Hepatitis A, (formerly known as infectious hepatitis) is an acute Infectious disease of the Liver caused by Hepatitis A virus which is Poliovirus, the causative agent of Poliomyelitis, is a human Enterovirus and member of the family of Picornaviridae. The Herpesviridae are a large family of DNA viruses that cause diseases in animals including humans A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. The Epstein-Barr Virus ( EBV) also called Human herpesvirus 4 (HHV-4 is a Virus of the herpes family (which includes Herpes Herpes simplex virus 1 and 2 ( HSV-1 and HSV-2) are two species of the herpes virus family Herpesviridae, which cause infections Cytomegalovirus (CMV (from the Greek cyto-, "cell" and -megalo-, "large" is a viral Kaposi's sarcoma-associated herpesvirus ( KSHV) is the eighth human Herpesvirus; its formal name according to the International Committee on Taxonomy of Viruses is Varicella zoster virus ( VZV) is one of eight herpes viruses known to infect humans (and other vertebrates Hepadnaviruses are a family of Viruses which can cause Liver infections in humans and animals A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. The Flaviviridae are a family of Viruses that are primarily spread through arthropod vectors (mainly ticks and mosquitoes An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. This page is for the virus For the disease see Hepatitis C. Hepatitis C virus ( HCV) is a small (50 nm in size A retrovirus is any Virus belonging to the viral family Retroviridae. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Human immunodeficiency virus ( HIV) is a Lentivirus (a member of the Retrovirus family that can lead to acquired immunodeficiency syndrome The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that Paramyxoviruses are Viruses of the Paramyxoviridae family of the Mononegavirales order they are negative-sense single-stranded An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Measles (rubeola is a Disease caused by a virus specifically a Paramyxovirus of the genus Morbillivirus. Mumps or epidemic Parotitis is a Viral disease of the Human species Human parainfluenza viruses (HPIVs are a group of four distinct Serotypes of single-stranded RNA Viruses belonging to the Paramyxovirus Human respiratory syncytial virus (RSV is a negative-sense single-stranded RNA virus of the family Paramyxoviridae, which includes A Papovaviridae is a member of the Papovaviridae family of Viruses Papovaviridae includes two genera: Papillomavirus and A DNA virus is a Virus that has DNA as its Genetic material and replicates using a DNA-dependent DNA polymerase. Papillomaviruses were first identified in the early 20th century when it was shown that skin Warts or Papillomas could be transmitted between individuals Rhabdoviruses are Viruses belonging to the family Rhabdoviridae, which is in the order Mononegavirales. An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. Rabies (from rabies “madness rage fury” Also known as “ hydrophobia ” is a viral Zoonotic neuroinvasive disease that Reoviridae is a family of Viruses that can affect the Gastrointestinal system (such as Rotavirus) and Respiratory tract Ribonucleic acid ( RNA) is a Nucleic acid that consists of a long chain of Nucleotide units Rotavirus is a Genus of Double-stranded RNA virus in the family Reoviridae. The Togaviridae are a family of Viruses including the following genera Genus Alphavirus; type species Sindbis virus An RNA virus is a Virus that has RNA (ribonucleic acid as its Genetic material. This page is for the virus For the disease see Rubella. Rubella virus is an enveloped, single-stranded RNA virus and the only Molecular biology is the study of Biology at a molecular level See also List of basic cell biology topics. Cell biology (also called cellular biology or formerly cytology, from the The study and use of viruses have provided valuable information about aspects of cell biology. For example, viruses have been useful in the study of genetics and helped our understanding of the basic mechanisms of molecular genetics, such as DNA replication, transcription, RNA processing, translation, protein transport, and immunology. Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is Molecular genetics is the field of Biology which studies the structure and function of Genes at a molecular level DNA replication is the process of copying a double-stranded DNA molecule to form two double-stranded molecules Transcription is the synthesis of RNA under the direction of DNA Post-transcriptional modification is a process in Cell biology by which in Eukaryotic cells, primary transcript RNA is converted into mature Translation is the first stage of Protein biosynthesis (part of the overall process of Gene expression) Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Immunology is a broad branch of biomedical Science that covers the study of all aspects of the Immune system in all Organisms It deals with

Gene therapy using an Adenovirus vector

Geneticists often use viruses as vectors to introduce genes into cells that they are studying. Gene therapy is the insertion of Genes into an individual's cells and tissues to treat a Disease, and Hereditary diseases in which a Adenoviruses are medium-sized (90–100 nm) Nonenveloped (naked Icosahedral viruses composed of a nucleocapsid and a double-stranded linear DNA Genetics (from Ancient Greek grc-Latn genetikos, “genitive” and that from grc-Latn genesis, “origin” a discipline of Biology, is In Epidemiology, a vector is an Organism that does not cause Disease itself but which transmits Infection by conveying Pathogens from This is useful for making the cell produce a foreign substance, or to study the effect of introducing a new gene into the genome. In similar fashion, virotherapy uses viruses as vectors to treat various diseases, as they can specifically target cells and DNA. Virotherapy is an experimental form of Cancer treatment using Biotechnology to convert Viruses into cancer-fighting agents by reprogramming viruses to only It shows promising use in the treatment of cancer and in gene therapy. Gene therapy is the insertion of Genes into an individual's cells and tissues to treat a Disease, and Hereditary diseases in which a Eastern European scientists have used phage therapy as an alternative to antibiotics for some time, and interest in this approach is increasing, due to the high level of antibiotic resistance now found in some pathogenic bacteria. Phage therapy is the Therapeutic use of Bacteriophages to treat Pathogenic Bacterial infections Antibiotic resistance is the ability of a Microorganism to withstand the effects of Antibiotics. ^[105]

Granulosis (GV) and nucleo-polyhedrosis viruses (NPV) may also be used as biological insecticides (e. Recent efforts to reduce the impact of broad-spectrum chemical Pesticides have brought biological insecticides back into vogue g. Cydia pomonella granulovirus). Cydia pomonella granulosis virus (CpGV is a Baculovirus Granulovirus.

Materials science and nanotechnology

Current trends in nanotechnology promise to make much more versatile use of viruses. From the viewpoint of a materials scientist, viruses can be regarded as organic nanoparticles. ^[106] Their surface carries specific tools designed to cross the barriers of their host cells. The size and shape of viruses, and the number and nature of the functional groups on their surface, is precisely defined. As such, viruses are commonly used in materials science as scaffolds for covalently linked surface modifications. A particular quality of viruses is that they can be tailored by directed evolution. The powerful techniques developed by life sciences are becoming the basis of engineering approaches towards nanomaterials, opening a wide range of applications far beyond biology and medicine. ^[107]

Because of their size, shape, and well-defined chemical structures, viruses have been used as templates for organizing materials on the nanoscale. Recent examples include work at the Naval Research Laboratory in Washington, DC, using Cowpea Mosaic Virus (CPMV) particles to amplify signals in microarray based sensors. The United States Naval Research Laboratory (NRL is the corporate research laboratory for the United States Navy and the United States Marine Corps and conducts a Washington DC ( formally the District of Columbia and commonly referred to as Washington, the District, or simply D The Cowpea mosaic virus is a plant Mosaic Virus of the comovirus group In this application, the virus particles separate the fluorescent dyes used for signaling in order to prevent the formation of non-fluorescent dimers that act as quenchers. Fluorescence is a Luminescence that is mostly found as an A dye can generally be described as a Colored substance that has an affinity to the substrate to which it is being applied A dimer is a Chemical or Biological entity consisting of two subunits called Monomers which are held together by either Intramolecular forces Quenching refers to any process which decreases the Fluorescence intensity of a given substance ^[108] Another example is the use of CPMV as a nanoscale breadboard for molecular electronics. ^[109] In April 2006, scientists at the Massachusetts Institute of Technology (MIT) created nanoscale metallic wires using a genetically-modified virus. Nanotechnology, sometimes shortened to nanotech, refers to a field of Applied science whose theme is the control of matter on an Atomic and Molecular Genetic engineering, Recombinant DNA technology, genetic modification/manipulation (GM and gene splicing are terms that apply to the direct ^[110] The MIT team was able to use the virus to create a working battery with an energy density up to three times more than current materials. In electronics a battery is a combination of two or more Electrochemical cells which store chemical Energy which can be converted into electrical energy Energy density is the amount of Energy stored in a given system or region of space per unit Volume, or per unit Mass, depending on the context although The potential exists for this technology to be used in liquid crystals, solar cells, fuel cells, and other electronics in the future. Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional Liquid, and those of a Solid A solar cell or photovoltaic cell is a device that converts Solar energy into Electricity by the photovoltaic effect. A fuel cell is an electrochemical conversion device It produces electricity from Fuel (on the Anode side and an oxidant (on the

Weapons

For more details on this topic, see Biological warfare. Biological warfare (BW — known as a germ warfare, biological weapons and bioweaponry — is the use of any Pathogen ( Bacterium

The ability of viruses to cause devastating epidemics in human societies has led to the concern that viruses could be weaponised for biological warfare. In Epidemiology, an epidemic (from Greek epi- upon + demos people is a classification of a disease that appears as new cases in a Biological warfare (BW — known as a germ warfare, biological weapons and bioweaponry — is the use of any Pathogen ( Bacterium Further concern was raised by the successful recreation of the infamous 1918 influenza virus in a laboratory. ^[111] The smallpox virus devastated numerous societies throughout history before its eradication. Smallpox is an Infectious disease unique to humans caused by either of two virus variants named Variola major and Variola minor. It currently exists in several secure laboratories in the world, and fears that it may be used as a weapon are not totally unfounded. The vaccine for smallpox is not safe, and during the years before the eradication of smallpox disease more people became seriously ill as a result of vaccination than did people from smallpox^[112] and smallpox vaccination is no longer universally practiced. ^[113] Thus, the modern global human population has almost no established resistance to smallpox; if it were to be released, a massive loss of life could be sustained before the virus is brought under control.

Electron micrographs of viruses

Norovirus. Norovirus (was "Norwalk-like viruses" an RNA virus of the Caliciviridae taxonomic family, causes approximately 90% of Epidemic This RNA virus causes winter vomiting disease. It is often in the news as a cause of gastro-enteritis on cruise ships and in hospitals.	Caliciviruses are related to Noroviruses.	Torovirus. An enveloped RNA virus.	Coronaviruses are a group of viruses that have a halo, or crown-like (corona) appearance when viewed under a microscope.
Ebola Virus is a filamentous RNA virus.	Measles virus. Measles (rubeola is a Disease caused by a virus specifically a Paramyxovirus of the genus Morbillivirus. This is called a thin section where the virus particle has been cut in two.	Respiratory Syncytial Virus (RSV). Human respiratory syncytial virus (RSV is a negative-sense single-stranded RNA virus of the family Paramyxoviridae, which includes In this preparation the ribonucleoprotein can be seen as a herring bone pattern.	Parvovirus B19. Parvovirus B19 ( B19 virus) was the first (and until 2005 the only known human Parvovirus. Parvovirus B19 is a small DNA virus best known for causing a childhood exanthema called fifth disease or erythema infectiosum.
Human Papilloma Virus	Influenza virus	Transmission electron micrograph of Herpes virus an enveloped virus that looks like fried eggs by negative stain electron microscopy. A human papillomavirus ( HPV) is a Papillomavirus that infects the skin and Mucous membranes of Humans Approximately 130 HPV types have been The Orthomyxoviridae (Derivation of name orthos is Greek for straight myxa is Greek for Mucus) are a family of RNA viruses that Herpes simplex is a Viral disease caused by Herpes simplex viruses both herpes simplex virus 1 (HSV-1 and herpes simplex virus 2 (HSV-2 cause herpes simplex	TEM micrograph of Poliovirus virions. A micrograph, microphotograph or photomicrograph is a Photograph or similar image taken through a Microscope or similar device to show Poliovirus, the causative agent of Poliomyelitis, is a human Enterovirus and member of the family of Picornaviridae.

See also

• Virology
• Influenza
• Rotavirus
• Herpes simplex virus
• Hepatitis B virus
• Satellite
• Neurotropic virus
• Bacteriophages (bacterial viruses)

Viruses templates

References