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 .