Virus

Virus, submicroscopic parasitic microorganism comprising a protein or protein/lipid sheath containing nucleic acid (DNA or RNA). Viruses are inert outside living cells, but within appropriate cells they can replicate (using raw material from the cell), causing viral diseases in the host organism. Various viruses infect animals, plants, and bacteria (in which case they are bacteriophages). Few drugs act specifically against viruses, although immunity can be induced against particular viruses. Various pathogenic organisms formerly regarded as large viruses are now distinguished as bedsonia.