What is the term for a virus that infects bacteria?

A virus that infects bacteria is called a bacteriophage, or simply a phage. The name combines the Greek words for “bacteria” and “eating,” reflecting the way many phages destroy their bacterial hosts.

A bacteriophage carries DNA or RNA inside a protein coat. It attaches to specific molecules on a bacterial cell, injects its genetic material, and uses the host’s machinery to make new phages. In the lytic cycle, the infected cell eventually bursts, releasing the new virus particles. In the lysogenic cycle, some phages insert their genes into the bacterial chromosome and remain dormant as prophages.

Phages are extraordinarily abundant wherever bacteria live, including soil, oceans, and the human body. They are also being studied as a possible treatment for antibiotic-resistant infections, a practice known as phage therapy. A common mix-up is calling a bacteriophage a bacterium; it is a virus, not a living bacterial cell.