n.
Microscopic, simple infectious agent that can multiply only in living cells of animals, plants, or bacteria.
Viruses are much smaller than bacteria and consist of a single-or double-stranded nucleic acid ( DNA or RNA ) surrounded by a protein shell called a capsid; some viruses also have an outer envelope composed of lipid s and proteins. They vary in shape. The two main classes are RNA viruses (see retrovirus ) and DNA viruses. Outside of a living cell, a virus is an inactive particle, but within an appropriate host cell it becomes active, capable of taking over the cell's metabolic machinery for the production of new virus particles ( virion s). Some animal viruses produce latent infections, in which the virus persists in a quiet state, becoming periodically active in acute episodes, as in the case of the herpes simplex virus. An animal can respond to a viral infection in various ways, including fever, secretion of interferon , and attack by the immune system. Many human diseases, including influenza , the common cold , and AIDS , as well as many economically important plant and animal diseases, are caused by viruses. Successful vaccine s have been developed to combat such viral diseases as measles , mumps , poliomyelitis , smallpox , and rubella . Drug therapy is generally not useful in controlling established viral infections, since drugs that inhibit viral development also inhibit the functions of the host cell. See also adenovirus ; arbovirus ; bacteriophage ; picornavirus ; plant virus ; poxvirus .