A bacteriophage, also known simply as a phage, is a virus that attacks and infects bacteria. The infection may or may not lead to the death of the bacterium, depending on the phage and sometimes on conditions. Each bacteriophage is specific to one form of bacterium.
Bacteriophages make up a diverse group of viruses, some of which have complex structures, including double-stranded DNA. Since the first was discovered in 1915, bacteriophages have been important in the study of genetics.