What is an infectious agent that can only replicate within the cells of a living host?

A virus is an infectious agent that can only replicate within the cells of a living host. Unlike bacteria, fungi, and protozoa which can often be found as single cells that replicate independently, viruses lack the cellular machinery necessary for reproduction and metabolism. They are composed of genetic material (either DNA or RNA) encased in a protein coat and require a host cell to replicate. Upon entering a host cell, viruses hijack the host's cellular machinery to produce copies of themselves, eventually leading to the host cell's damage or death. This unique dependency on a living host for replication is a defining characteristic of viruses.

In contrast, the other options represent organisms that can reproduce outside of a host. Bacteria are single-celled organisms that can grow and reproduce independently in various environments. Fungi can also thrive on their own, decomposing organic matter, and protozoa are unicellular organisms that can survive and replicate independently in a range of habitats. Thus, the defining trait of viruses being obligate intracellular parasites sets them apart as the correct answer.