Sometimes shortened to ‘phage’, this is a VIRUS which invades a BACTERIUM. Containing either single-stranded or double-stranded DNA or RNA, a particular phage generally may infect one or a limited number of bacterial strains or species. When nucleic acid from a bacteriophage enters a bacterium, it programmes it to start manufacturing virus components, which are assembled into virus particles and released when the bacteria lyse (disintegrate). Other (temperate) phages induce a non-lytic, or lysogenic, state, in which phage nucleic acid integrates stably into, and replicates with, the bacterial chromosome. This relationship can revert to a lytic cycle and production of new phages. In the process the phage may carry small amounts of bacterial DNA to a new host: the production of diphtheria toxin by Corynebacterium diphtheriae and of erythrogenic toxin by Streptococcus pyogenes are well-known examples of this effect. Phages have been used in experimental treatment of certain rare metabolic disorders in the hope that they will carry into body cells the enzyme or other chemical which is missing in the patient.