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.
RNA viruses include HIV and hepatitis C virus. Researchers have grouped viruses together into several major families, based on their shape, behavior, and other characteristics. These include the herpes-viruses, adenoviruses, papovaviruses (including
the papillomaviruses), hepadnaviruses, poxviruses, and parvoviruses, among the DNA viruses. On the RNA virus side, major families include the picorna-viruses (including the rhinoviruses), calciviruses, paramyxoviruses, orthomyxoviruses, rhabdoviruses, filoviruses, and retroviruses. There are dozens
of smaller virus families within these major classifications. Many viruses are host specific, capable of infecting and causing disease in humans or specific animals only.
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