Bacteriophages are viruses that depend on bacteria as host organisms for their propagation.
Phages are composed of genetic material and a protein envelope only. They cannot propagate without their bacterial host. Once they found a matching host, they make him replicate their genetic material. New phages are produced and, after disruption of the bacterial cell wall, released. Due to their specificity for certain bacteria, phages represent an alternative to antibiotics. Phages are already being used in gene technology, for instance.