Bacteriophage is a virus that infects bacteria. The term is usually used as a phage, which is in its abbreviated form.
Like viruses infecting eukaryotes (animals, plants, and fungi), phages also have a large structural and functional diversity. They typically consist of a shell of protein and the genetic material contained in it. Genetic material can be DNA or RNA, but it usually consists of double – stranded DNA with a base pair length of 5 to 500 pounds. Bacteriophages are usually between 20 and 200 nm in size.
Phages are ubiquitous and can be found in environments where bacteria live, for example in soil or in animal intestines. One of the most intense natural sources of phages and other viruses is seawater. 109 active phage grains (virions) per milliliter have been found on the sea surface, and 70% of marine bacteria may have been infected by phages.