What is the term for a virus that infects bacteria?

The term for a virus that infects bacteria is "bacteriophage." Bacteriophages are a specific type of virus that target and invade bacterial cells, utilizing the bacterial machinery to replicate and produce new virus particles. Once they enter a bacterium, they can either enter a lytic cycle where the bacterium is destroyed, or a lysogenic cycle where the viral DNA integrates into the bacterial genome and can remain dormant for periods of time.

The other terms mentioned refer to different types of viruses or infectious agents. A retrovirus, for instance, is a type of virus that uses reverse transcriptase to convert its RNA genome into DNA, which then integrates into the host's genome, but it specifically targets animal cells. A phytovirus refers to viruses that infect plants, while a viroid is a small circular RNA molecule that can cause diseases in plants but is not classified as a virus. Thus, the association of bacteriophages with bacteria is what makes this term the correct answer.