From Wikipedia, the free encyclopedia
| Virus | |
|---|---|
| Rotavirus | |
| Virus classification | |
| (unranked): | Virus |
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| See text |
A virus is a small infectious agent that replicates only inside the living cells of an organism. Viruses can infect all types of life forms, from animals and plants to microorganisms, including bacteria and archaea.[1]
Since Dmitri Ivanovsky‘s 1892 article describing a non-bacterial pathogen infecting tobacco plants, and the discovery of the tobacco mosaic virus by Martinus Beijerinck in 1898,[2] about 5,000 virus species have been described in detail,[3] of the millions of types of viruses in the environment.[4] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.[5][6] The study of viruses is known as virology, a sub-speciality of microbiology.
While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or virions, consisting of: (i) the genetic material, i.e. long molecules of DNA or RNA that encode the structure of the proteins by which the virus acts; (ii) a protein coat, the capsid, which surrounds and protects the genetic material; and in some cases (iii) an outside envelope of lipids. The shapes of these virus particles range from simple helical and icosahedral forms to more complex structures. Most virus species have virions too small to be seen with an optical microscope, about one hundredth the size of most bacteria.
The origins of viruses in the evolutionary history of life are unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of horizontal gene transfer, which increases genetic diversity in a way analogous to sexual reproduction.[7] Viruses are considered by some to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack key characteristics (such as cell structure) that are generally considered necessary to count as life. Because they possess some but not all such qualities, viruses have been described as “organisms at the edge of life”,[8] and as replicators.[9]
Viruses spread in many ways. One transmission pathway is through disease-bearing organisms known as vectors: for example, viruses are often transmitted from plant to plant by insects that feed on plant sap, such as aphids; and viruses in animals can be carried by blood-sucking insects. Influenza viruses are spread by coughing and sneezing. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by contact and entering the body in food or water. HIV is one of several viruses transmitted through sexual contact and by exposure to infected blood. The variety of host cells that a virus can infect is called its “host range“. This can be narrow, meaning a virus is capable of infecting few species, or broad, meaning it is capable of infecting many.[10]
Viral infections in animals provoke an immune response that usually eliminates the infecting virus. Immune responses can also be produced by vaccines, which confer an artificially acquired immunity to the specific viral infection. Some viruses, including those that cause AIDS, HPV, and viral hepatitis, evade these immune responses and result in chronic infections. Several antiviral drugs have been developed.
Etymology
The word is from the Latin neuter vīrus referring to poison and other noxious liquids, from the same Indo-European base as Sanskrit viṣa, Avestan vīša, and ancient Greek ἰός (all meaning “poison”), first attested in English in 1398 in John Trevisa’s translation of Bartholomeus Anglicus’s De Proprietatibus Rerum.[11][12] Virulent, from Latin virulentus (poisonous), dates to c. 1400.[13][14] A meaning of “agent that causes infectious disease” is first recorded in 1728,[12] long before the discovery of viruses by Dmitri Ivanovsky in 1892. The English plural is viruses (sometimes also viri[15] or vira[16]), whereas the Latin word is a mass noun, which has no classically attested plural (vīra is used in Neo-Latin[17]). The adjective viral dates to 1948.[18] The term virion (plural virions), which dates from 1959,[19] is also used to refer to a single viral particle that is released from the cell and is capable of infecting other cells of the same type.[20]
History
Martinus Beijerinck in his laboratory in 1921
Main articles: History of virology and Social history of viruses
Louis Pasteur was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected by microscopes.[21] In 1884, the French microbiologist Charles Chamberland invented the Chamberland filter (or Pasteur-Chamberland filter) with pores small enough to remove all bacteria from a solution passed through it.[22] In 1892, the Russian biologist Dmitri Ivanovsky used this filter to study what is now known as the tobacco mosaic virus: crushed leaf extracts from infected tobacco plants remained infectious even after filtration to remove bacteria. Ivanovsky suggested the infection might be caused by a toxin produced by bacteria, but did not pursue the idea.[23] At the time it was thought that all infectious agents could be retained by filters and grown on a nutrient medium—this was part of the germ theory of disease.[2] In 1898, the Dutch microbiologist Martinus Beijerinck repeated the experiments and became convinced that the filtered solution contained a new form of infectious agent.[24] He observed that the agent multiplied only in cells that were dividing, but as his experiments did not show that it was made of particles, he called it a contagium vivum fluidum (soluble living germ) and re-introduced the word virus. Beijerinck maintained that viruses were liquid in nature, a theory later discredited by Wendell Stanley, who proved they were particulate.[23] In the same year Friedrich Loeffler and Paul Frosch passed the first animal virus through a similar filter: aphthovirus, the agent of foot-and-mouth disease.[25]
In the early 20th century, the English bacteriologist Frederick Twort discovered a group of viruses that infect bacteria, now called bacteriophages[26] (or commonly ‘phages’), and the French-Canadian microbiologist Félix d’Herelle described viruses that, when added to bacteria on an agar plate, would produce areas of dead bacteria. He accurately diluted a suspension of these viruses and discovered that the highest dilutions (lowest virus concentrations), rather than killing all the bacteria, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the original suspension.[27] Phages were heralded as a potential treatment for diseases such as typhoid and cholera, but their promise was forgotten with the development of penicillin. The development of bacterial resistance to antibiotics has renewed interest in the therapeutic use of bacteriophages.[28]
By the end of the 19th century, viruses were defined in terms of their infectivity, their ability to pass filters, and their requirement for living hosts. Viruses had been grown only in plants and animals. In 1906, Ross Granville Harrison invented a method for growing tissue in lymph, and, in 1913, E. Steinhardt, C. Israeli, and R. A. Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue.[29] In 1928, H. B. Maitland and M. C. Maitland grew vaccinia virus in suspensions of minced hens’ kidneys. Their method was not widely adopted until the 1950s when poliovirus was grown on a large scale for vaccine production.[30]
Another breakthrough came in 1931, when the American pathologist Ernest William Goodpasture and Alice Miles Woodruff grew influenza and several other viruses in fertilised chicken eggs.[31] In 1949, John Franklin Enders, Thomas Weller, and Frederick Robbins grew polio virus in cultured human embryo cells, the first virus to be grown without using solid animal tissue or eggs. This work enabled Jonas Salk to make an effective polio vaccine.[32]
The first images of viruses were obtained upon the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll.[33] In 1935, American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it was mostly made of protein.[34] A short time later, this virus was separated into protein and RNA parts.[35] The tobacco mosaic virus was the first to be crystallised and its structure could, therefore, be elucidated in detail. The first X-ray diffraction pictures of the crystallised virus were obtained by Bernal and Fankuchen in 1941. On the basis of her pictures, Rosalind Franklin discovered the full structure of the virus in 1955.[36] In the same year, Heinz Fraenkel-Conrat and Robley Williams showed that purified tobacco mosaic virus RNA and its protein coat can assemble by themselves to form functional viruses, suggesting that this simple mechanism was probably the means through which viruses were created within their host cells.[37]
The second half of the 20th century was the golden age of virus discovery and most of the over 2,000 recognised species of animal, plant, and bacterial viruses were discovered during these years.[38] In 1957, equine arterivirus and the cause of Bovine virus diarrhoea (a pestivirus) were discovered. In 1963, the hepatitis B virus was discovered by Baruch Blumberg,[39] and in 1965, Howard Temin described the first retrovirus. Reverse transcriptase, the enzyme that retroviruses use to make DNA copies of their RNA, was first described in 1970, independently by Howard Martin Temin and David Baltimore.[40] In 1983 Luc Montagnier‘s team at the Pasteur Institute in France, first isolated the retrovirus now called HIV.[41] In 1989 Michael Houghton‘s team at Chiron Corporation discovered Hepatitis C.[42][43]
More at: https://en.wikipedia.org/wiki/Virus
