Viruses - defined as a piece of nucleic acid surrounded by bad news by Peter Medawar, a Nobel prize-winning biologist - are obligate intracellular parasites. Viruses can infect all cellular forms of life from bacteria to plants to animals. Outside of a living cell, viruses are inanimate particles - no more biologically active than a crystal of salt. Once inside a living host cell, however, virus particles are able to pirate the host cell's own machinery to reproduce. Since viruses are incapable of
any independent metabolism, many biologists and chemists do not consider them to be living entities. The fact that they do have a replicating form, however, has led to a lively debate over this classification which is still not truly resolved.
An excellent all-encompassing site to learn more is
All the Virology of the WWW.
Some of the features of this site are: online virology courses, course notes and tutorials; the Big Picture Book of Viruses; virology dictionaries and glossaries; daily virology news from over 200 newsfeeds; lots and lots of cool links and even a graffiti wall. Definitely recommended!