What is a microorganism capable of replicating within living cells?

A microorganism that is capable of replicating within living cells is classified as a virus. Viruses are unique among microorganisms because they do not possess the cellular machinery required for replication on their own. Instead, they must invade a host cell and hijack the cell's biological processes to produce new virus particles. This dependence on living cells for replication distinguishes viruses from other microorganisms such as fungi, bacteria, and protozoa, which can often reproduce independently.

Fungi, bacteria, and protozoa are all cellular organisms that can survive and replicate outside of host cells. They have their own cellular structures and metabolic pathways, allowing them to reproduce in various environments, including soil, water, and within living hosts. However, these organisms do not require a host cell to replicate as viruses do, which is why the definition specifically points to viruses as the correct answer.