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A phage (also called bacteriophage) is a small virus that infects only bacteria. Like viruses, they consist of an outer protein hull and the enclosed genetic material (which consists of dsDNA in 95% of the phages known) of 20-650 kbp (kilo base pairs). Phages were discovered independently by Frederick Twort in 1915 and by Félix D’Herelle in 1917.

Phages infect only specific bacteria, and many are virulent phages, meaning they reproduce within the infected bacterium, then lyse (destroy) it so the new phages are released. <note>A famous quote from the microbiologist Mark Müller says: Bacteria don't die, they just phage away.</note> Some phages (so-called temperate phages), though, integrate their genetic material into the DNA of the host bacterium and stay dormant, similar to endogenic retroviruses in animals. These endogenic phages are then copied with every cell division together with the DNA of the host cell. They do not harm the cell, but monitor (via some proteins they code for) the status of their host. When the host cell shows signs of stress (meaning it might be about to die soon), the endogenic phages become active again and start their reproduction cycle, resulting in the lysis of the host cell. For this reason, these phages have also been called lysogenic phages. An example is phage λ in E. coli. Sometimes, endogenic phages even benefit the host bacterium while they are dormant by adding new functions to the bacterial genome. A famous example is the harmless Vibrio bacteria strain, which is turned into Vibrio cholerae by a phage, causing cholera.

Phages play an important role in molecular biology as cloning vectors to insert DNA into bacteria. They are also being evaluated as a medical treatment against bacterial infections--because killing bacteria is what phages do best.