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Virus

From Tilcayo Cats, the lean encyclopaedia by Tilcayo Knowledge Systems
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A virus is a very small, non-cellular parasite of cells. Its genetic material, which is made of either DNA or RNA, is enclosed in a protein coat.[1] Viruses can only make copies of themselves inside the cells of a living host, because they depend on the host cell for building materials, machinery and energy.[2][3]

Viruses are thought to infect every type of living thing, including animals, plants, fungi, bacteria and archaea.[4][2] Most viruses do not cause disease in humans, and viruses also play a part in recycling nutrients in nature.[4] The viruses discovered so far are probably only a tiny fraction of those on Earth.[2]

Discovery

In the late 19th century two scientists working independently, Dimitri Ivanovski in Russia and Martinus Beijerinck in Holland, found evidence for very small infectious agents.[5] They made extracts from diseased tobacco plants and passed them through filters fine enough to hold back bacteria. The filtered liquid could still infect new plants, but no bacteria could be grown from it.[5][3] Ivanovski's work in 1892 is seen as the start of virology, the study of viruses.[3] Beijerinck called the agent a "virus", and the term has been used ever since.[5] The agent is now known as tobacco mosaic virus.[5]

In 1901 Walter Reed's team showed that yellow fever is caused by a virus and spread by mosquitoes.[3] Viruses that infect bacteria, called bacteriophages or phages, were discovered independently by Frederick Twort in 1915 and Félix d'Herelle in 1917.[6][2]

Structure

A virus particle is called a virion. Most virions are between about 10 and 100 nanometres across, much smaller than bacteria, which are usually 1 to 10 micrometres long.[7] Most can only be seen with an electron microscope.[8] A few giant viruses are much larger: mimivirus has a protein coat about 400 nanometres across.[9]

The genetic material of a virus may be DNA or RNA.[7] It is enclosed in a protein coat called a capsid, which protects it until it can be delivered into a cell.[10][10] Capsids are built from many copies of one or a few kinds of protein.[11] Many have one of two shapes: a helical rod, as in tobacco mosaic virus, or an icosahedron, a shape with 20 faces.[12][13] Most known phages have a head containing the genetic material, attached to a tail.[14][6]

Some viruses, especially many animal viruses, also have an outer membrane called an envelope, which they take from the host cell.[3][15] The envelope is fragile, so viruses without one are generally tougher.[16]

How viruses multiply

Viruses are usually very specific about which species and cell types they infect.[17] To multiply, a virus attaches to a host cell, gets inside, makes copies of its genetic material and proteins, assembles new virions and then leaves the cell.[18][18] Unlike a cell, a new virion is never formed directly from an existing virion. Instead, it is put together from parts made separately in the host cell.[1][19] Some viruses leave by budding out through the cell membrane, while others build up inside the cell until it bursts.[20][6]

Are viruses alive?

Scientists continue to debate whether viruses are living, and the answer depends on how life is defined.[21] Viruses have genes that are copied inside cells. However, outside a host cell they are essentially inert, with no metabolism of their own.[22][19] They do not respond to their surroundings or grow like cells.[19] For these reasons, some textbooks do not count them as microorganisms. Instead, they use the wider term "microbes" to cover both microorganisms and viruses.[23]

Viruses in history

Smallpox, a disease caused by a virus, troubled humans for thousands of years. It probably caused the death of Ramses V. The last natural case occurred in 1977, and in 1980 the world was declared free of smallpox.[3] Work on viruses has also helped science more widely. In 1952 Hershey and Chase used a phage to provide strong evidence that genetic material is DNA.[5]

See also

References

  1. ↑ 1.0 1.1 Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 6. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  2. ↑ 2.0 2.1 2.2 2.3 Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 2. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 138. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  4. ↑ 4.0 4.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 137. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  5. ↑ 5.0 5.1 5.2 5.3 5.4 Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 4. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  6. ↑ 6.0 6.1 6.2 Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 238. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  7. ↑ 7.0 7.1 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 139. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  8. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. pp. 4–5. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  9. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 140. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  10. ↑ 10.0 10.1 Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 5. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  11. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 35. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  12. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 141. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  13. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 36. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  14. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 39. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  15. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. pp. 41–42. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  16. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 853. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  17. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 861. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  18. ↑ 18.0 18.1 Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 56. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  19. ↑ 19.0 19.1 19.2 Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 10. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
  20. ↑ Zedalis, Julianne; Eggebrecht, John (2018). Biology for AP Courses. Houston: OpenStax. p. 863. ISBN 978-1-947172-40-1. Retrieved 2026-10-23.
  21. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. p. 7. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  22. ↑ Carter, John B.; Saunders, Venetia A. (2013). Virology: Principles and Applications (2nd ed.). Chichester: John Wiley & Sons. pp. 6–7. ISBN 978-1-119-99142-7. {{cite book}}: |access-date= requires |url= (help)
  23. ↑ Wessner, David R.; Dupont, Christine; Charles, Trevor C.; Neufeld, Josh D. (2021). Microbiology (3rd ed.). Hoboken, NJ: Wiley. p. 3. ISBN 978-1-119-71581-8. Retrieved 2026-10-23.
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