Which type of infectious agent is capable of living and reproducing within other cells?

Viruses are unique among infectious agents because they require a host cell to live and reproduce. Unlike bacteria, fungi, and parasites, which can often survive and multiply independently in a variety of environments, viruses cannot replicate on their own. They consist of genetic material (either DNA or RNA) encased in a protein coat and must invade a living cell to hijack that cell's machinery for reproduction. Once inside a host cell, viruses can replicate and produce new viral particles, which can then go on to infect other cells. This dependency on host cells for reproduction is a defining characteristic of viruses, distinguishing them from other types of infectious agents.