viruses can be classified on the basis of the hosts they infect as well as by their genomes. Thus, we have bacterial viruses, archaeal viruses, animal viruses, plant viruses, and viruses that infect other kinds of eukaryotic cells. Bacterial viruses, sometimes called bacteriophages (or phage for short;from the Greek phagein,meaning "to eat") have been intensively studied as model systems for the molecular biology and genetics of virus replication. Species of both Bacteria and Archaea are infected by specific viruses. Indeed, many of the basic concepts of virology were first worked out with bacterial viruses and subsequently applied to viruses of higher organisms. Because of their frequent medical importance, animal viruses have been extensively studied, whereas plant viruses,although of enormous importance to modern agriculture,have been less well studied.