Viruses are microbial parasites that insert information into the cell that they have infected to divert the host synthetic systems to make viral rather than cellular components. The process is absolutely dependent on living cells as viruses have no metabolic machinery themselves. The virus consists of a replicating unit or replicon, and a packaging system to deliver the virus to the cell; if either of these essential but distinct elements is missing the virus cannot multiply. All living things have their own viruses. New methods of virus discovery based on nucleic acid detection rather than infection of cells or disease in organisms are revealing more and more forms, many of which are not yet linked to any disease or disorder. The intimate connection between the cell and the virus makes it possible to study cellular processes in detail but also means that treating virus infections is not straightforward.

Key Concepts:

  • Viruses are intracellular parasites that require the machinery of living cells to replicate.

  • They consist of an informational molecule (RNA or DNA) in a delivery package that enables them to move from cell to cell.

  • They have been identified by the diseases they cause, their ability to infect cell cultures and increasingly by purely molecular methods such as deep sequencing.

  • The origin of viruses is not known.

  • Viruses replicate by subverting the cell machinery to the manufacture of virus by subtle means.

  • Some are the causative agents of devastating diseases yet equally some seem to have no ill effects.

  • The intimate relationship between virus and cell means that they can be used to investigate the details of cellular processes but they are difficult to attack.

Keywords: origin; replication; virus discovery; virions; intracellular parasites; virus‐host cell interactions; virus replication

Figure 1.

Nonenveloped viruses. In order of increasing size: (a) Poliovirus – causes paralysis, 27 nm in diameter; (b) Human papilloma virus – causes warts, 40 nm in diameter and (c) Adenovirus – causes colds or diarrhoea, 80–110 nm. Scale bar=100 nm.

Figure 2.

Enveloped viruses. In order of increasing size: (a) HIV causes AIDS, 100 nm in diameter; (b) Influenza virus, 100 nm in diameter; (c) Measles virus, 150 nm in diameter; (d) Rabies virus, 160 nm long, bullet shaped and (e) Herpes virus, 200 nm long, complex internal structure. Scale bar=100 nm.

Figure 3.

Diagrammatic representation of (a) a nonenveloped (icosahedral) virus and (b) an enveloped virus.

Figure 4.

The virus life cycle.



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Further Reading

ICTV (2012) Virus taxonomy. 2012 release. Accessed on 6 September 2013.

Knipe DM and Howley P (eds) (2013) Fields Virology, 6th edn, vol. I and II. Philadelphia, Baltimore, New York, London, Buenos Aires, Hong Kong, Sydney, Tokyo: Lippincott Williams and Wilkins. ISBN 9781451105636.

Mahy BWJ and Van Regenmortel MHV (eds) (2010) Desk Encyclopedia of Plant and Fungal Virology. Amsterdam, Boston, Heidelberg, London, New York, Oxford, Pais, San Diego, San Francisco, Singapore, Sydney, Tokyo: Academic Press. ISBN 978‐0123751485.

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Minor, Philip D(Jan 2014) Viruses. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000441.pub3]