What is a virus that infects bacteria known as?

A virus that specifically infects bacteria is known as a bacteriophage. Bacteriophages, often referred to simply as phages, are unique in that they can attach to bacterial cells and introduce their genetic material into the host, leading to the replication of new virus particles and potentially the lysis (breaking open) of the bacterial cell. This interaction plays a crucial role in regulating bacterial populations in various environments and can be leveraged in applications such as phage therapy to treat bacterial infections.

While retroviruses are known for infecting animal cells and integrating their genetic material into the host's genome, and prions are infectious proteins that cause misfolding of other proteins, neither of these categories includes viruses that target bacteria. Virophages, although they do exist, infect other viruses rather than directly targeting bacteria, thus the term bacteriophage remains the correct answer for a virus that infects bacterial cells.