virus (medicine)

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Infectious particle consisting of a core of nucleic acid (DNA or RNA) enclosed in a protein shell. They are extremely small and cause disease. They differ from all other forms of life in that they are not cells – they are acellular. They are able to function and reproduce only if they can invade a living cell to use the cell's system to replicate themselves. In the process they may disrupt or alter the host cell's own DNA. They use the cell they invade to make more virus particles that are then released. This usually kills the cell. The healthy human body reacts by producing an antiviral protein, interferon, which prevents the infection spreading to adjacent cells. There are around 5,000 species of virus known to science (1998), although there may be as many as half a million actually in existence.

Examples of diseases in humans caused by viruses are the common cold, chickenpox, influenza, AIDS, herpes, mumps, measles, and rubella. Recent evidence implicates viruses in the development of some forms of cancer (see oncogenes). Antibiotics do not work against viruses. The best protection against diseases caused by viruses is immunization.

Viruses can change by mutation. When they do so, a human body is sometimes unable to fight the new virus very well. This happens regularly with the influenza virus. A small change results in a small influenza epidemic, but a big change results in a pandemic that can kill millions of people worldwide. Many viruses mutate continuously so that the host's body has little chance of developing permanent resistance; others transfer between species, with the new host similarly unable to develop resistance. The viruses that cause AIDS and Lassa fever are both thought to have ‘jumped’ to humans from other mammalian hosts.

The ‘Sun of May’ was added in 1818. The blue bands are a shade known as ‘celeste’, said to be the colour of the sky which inspired Argentine revolutionary Manuel Belgrano before battle. Effective date: 16 August 1985. >>