A virus that is parasitic within a bacterium. Each phage is specific for only one type of bacterium. Most phages (virulent phages) infect, quickly multiply within, and destroy (lyse) their host cells. However, some (temperate phages) remain dormant in their hosts after initial infection: their nucleic acid becomes integrated into that of the host and multiplies with it, producing infected daughter cells (see lysogeny). Lysis may eventually be triggered by environmental factors. Phages are used experimentally to identify bacteria, to control manufacturing processes (such as cheese production) that depend on bacteria, and, because they can alter the genetic make-up of bacterial cells, they are important tools in genetic engineering as cloning vectors. Phage therapy is the targeted application of phages to treat bacterial infection. It was first used in 1919 by the French physician Félix d’Hérelle and subsequently developed especially in the Soviet Union and eastern Europe; in the West it was superseded by the introduction of antibiotics in the 1940s. The emergence of antibiotic-resistant strains of bacteria has prompted renewed interest in phage therapy since the 1990s, especially using bioengineered phages and phage-derived lytic proteins capable of destroying pathogen bacterial cells. See lambda phage.

https://www.asm.org/division/m/M.html Overview of phage biology and diversity sponsored by the American Society of Microbiology