Medical Definition of Virus
Virus: A microorganism that is smaller than a bacterium that cannot grow or reproduce apart from a living cell. A virus invades living cells and uses their chemical machinery to keep itself alive and to replicate itself. It may reproduce with fidelity or with errors (mutations); this ability to mutate is responsible for the ability of some viruses to change slightly in each infected person, making treatment difficult. Viruses cause many common human infections and are also responsible for a number of rare diseases. Examples of viral illnesses range from the common cold, which can be caused by one of the rhinoviruses, to AIDS, which is caused by HIV. Viruses may contain either DNA or RNA as their genetic material. Herpes simplex virus and the hepatitis B virus are DNA viruses. RNA viruses have an enzyme called reverse transcriptase that permits the usual sequence of DNA-to-RNA to be reversed so that the virus can make a DNA version of itself.
Last Editorial Review: 5/13/2016
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