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Virus

This article is about the type of pathogen. For the type of malware, see Computer virus. For other uses, see Virus (disambiguation).

For a more accessible and less technical introduction to this topic, see Introduction to viruses.

Virus
"Severe acute respiratory syndrome coronavirus 2"
SARS-CoV-2, a member of the subfamily Coronavirinae
Virus classification Edit this classification
(unranked): Virus
Realms

A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism.[1] Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea.[2][3] Viruses are found in almost every ecosystem on Earth and are the most numerous type of biological entity.[4][5] 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,[6]: 4  more than 11,000 of the millions of virus species have been described in detail.[7][8] The study of viruses is known as virology, a subspeciality of microbiology.

When infected, a host cell is often forced to rapidly produce thousands of copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent viral particles, or virions, consisting of (i) 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 and are one-hundredth the size of most bacteria.

The origins of viruses in the evolutionary history of life are still unclear. Some viruses may have evolved from plasmids, which are pieces of DNA that can move between cells. Other viruses 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.[9] Viruses are considered by some biologists to be a life form, because they carry genetic material, reproduce, and evolve through natural selection, although they lack some key characteristics, such as cell structure, that are generally considered necessary criteria for defining life. Because they possess some but not all such qualities, viruses have been described as "organisms at the edge of life"[10] and as replicators.[11]

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. Many viruses spread in the air by coughing and sneezing, including influenza viruses, SARS-CoV-2, chickenpox, smallpox, and measles. Norovirus and rotavirus, common causes of viral gastroenteritis, are transmitted by the faecal–oral route, passed by hand-to-mouth contact or in food or water. The infectious dose of norovirus required to produce infection in humans is fewer than 100 particles.[12] 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 is narrow for viruses specialized to infect only a few species, or broad for viruses capable of infecting many.[13]: 123–124 

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 HIV/AIDS, HPV infection, and viral hepatitis, evade these immune responses and result in chronic infections. Several classes of antiviral drugs have been developed.

  1. ^ Wu KJ (15 April 2020). "There are more viruses than stars in the universe. Why do only some infect us? – More than a quadrillion quadrillion individual viruses exist on Earth, but most are not poised to hop into humans. Can we find the ones that are?". National Geographic Society. Archived from the original on 15 April 2020. Retrieved 18 May 2020.
  2. ^ Koonin EV, Senkevich TG, Dolja VV (September 2006). "The ancient Virus World and evolution of cells". Biology Direct. 1 (1): 29. doi:10.1186/1745-6150-1-29. PMC 1594570. PMID 16984643.
  3. ^ Zimmer C (26 February 2021). "The Secret Life of a Coronavirus - An oily, 100-nanometer-wide bubble of genes has killed more than two million people and reshaped the world. Scientists don't quite know what to make of it". The New York Times. Archived from the original on 28 December 2021. Retrieved 28 February 2021.
  4. ^ Cite error: The named reference Lawrence_2009 was invoked but never defined (see the help page).
  5. ^ Edwards RA, Rohwer F (June 2005). "Viral metagenomics". Nature Reviews. Microbiology. 3 (6): 504–10. doi:10.1038/nrmicro1163. PMID 15886693. S2CID 8059643.
  6. ^ Cite error: The named reference Dimmock_2007 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference ictv_2022 was invoked but never defined (see the help page).
  8. ^ Breitbart M, Rohwer F (June 2005). "Here a virus, there a virus, everywhere the same virus?". Trends in Microbiology. 13 (6): 278–84. doi:10.1016/j.tim.2005.04.003. PMID 15936660.
  9. ^ Cite error: The named reference Canchaya_2003 was invoked but never defined (see the help page).
  10. ^ Rybicki EP (1990). "The classification of organisms at the edge of life, or problems with virus systematics". South African Journal of Science. 86: 182–86.
  11. ^ Cite error: The named reference Koonin_2016 was invoked but never defined (see the help page).
  12. ^ Robilotti E, Deresinski S, Pinsky BA (January 2015). "Norovirus". Clinical Microbiology Reviews. 28 (1): 134–64. doi:10.1128/CMR.00075-14. PMC 4284304. PMID 25567225.
  13. ^ Cite error: The named reference Shors_2017 was invoked but never defined (see the help page).

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