What type of microorganisms can only function inside living cells?

Viruses are unique in that they can only replicate and function within the host cells of living organisms. Unlike other microorganisms, such as fungi, bacteria, and protozoa, viruses lack the cellular machinery necessary for metabolic processes or reproduction on their own. They consist of genetic material, either DNA or RNA, enclosed in a protein coat and sometimes an outer lipid envelope.

To reproduce, a virus must invade a living cell and hijack the host's cellular mechanisms. Once inside, the viral genetic material takes control of the host's cell machinery to produce new virus particles. This reliance on a host organism for replication is what fundamentally distinguishes viruses from other microorganisms, which can typically grow and divide independently in a suitable environment.