A phage (also called bacteriophage) is a small virus that infectss only bacteria. Like viruses that infect eukaryotes, they consist of an outer protein hull and the enclosed genetic material (which consists of double-stranded DNA in 95% of the phages known) of 5-650 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. Some phages are virulent, meaning that upon infecting a cell they immediately begin reproducing, and within a short time lyse (destroy) the cell, releasing new phages. (A famous quote from the microbiologist Mark Müller says: Bacteria don't die, they just phage away.) Some phages (so-called temperate phages), though, can instead enter a relatively harmless state, either integrating their genetic material into the chromosomal DNA of the host bacterium (much like endogenous retroviruseses in animals) or establishing themselves as plasmids. These endogenous phages, referred to as prophages, are then copied with every cell division together with the DNA of the host cell. They do not kill 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 endogenous phages become active again and start their reproductive cycle, resulting in the lysis of the host cell. For this reason, these phages have also been called lysogenic phages. An example is phage &lambda of E. coli. Sometimes, prophages 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 by medical researchers as an alternative to antibiotics for treating bacterial infections—because killing bacteria is what phages do best.