Which infective agents require a host cell to replicate?

Viruses are unique among infectious agents because they are obligate intracellular parasites, meaning they rely entirely on a host cell for replication. Unlike other microorganisms, viruses lack the cellular machinery necessary for reproduction, such as ribosomes and metabolic pathways. Instead, they hijack the host cell's machinery to replicate their genetic material and produce new virus particles.

When a virus infects a host cell, it injects its genetic material into the cell, which then uses the host's resources to make copies of the viral genome and assemble new virions. This process typically culminates in the release of new viruses that can go on to infect additional cells, thereby perpetuating the viral lifecycle.

In contrast, protozoa, fungi, and bacteria are all capable of independent growth and reproduction outside of a host cell. Protozoa are single-celled eukaryotes that can reproduce asexually and often have a complex life cycle. Fungi can reproduce through spores and can live independently in various environments. Bacteria, being prokaryotic, can divide and thrive in diverse settings without a host. Thus, while these organisms have their own methods of replication and may cause infections, they do not require a host cell for their replication as viruses do.