What type of viruses infect bacteria?

Bacteriophages are specialized viruses that infect bacteria. They have a unique structure typically consisting of a head that contains the viral genome and a tail that allows them to attach to specific bacterial surfaces. Once attached, bacteriophages can inject their genetic material into the bacterial cell, leading to the replication of new virus particles and eventually causing the lysis of the bacterial cell. This process plays a significant role in controlling bacterial populations and is a fundamental component of microbial ecology.

The other types of viruses listed target different organisms. Riboviruses primarily infect plant and animal cells, and retroviruses are known for their ability to infect animal cells and integrate their RNA genome into the host's DNA, as seen with human immunodeficiency virus (HIV). Oncoviruses are a subset of viruses capable of causing cancer in animals and humans, but they do not specifically target bacteria. Thus, the distinct association of bacteriophages with bacterial infection highlights why this is the correct answer.