Which of the following infectious agents typically requires a host for replication?

Viruses are unique among infectious agents in that they absolutely require a host cell for replication. Unlike bacteria, fungi, and helminths, viruses cannot reproduce independently; they lack the necessary cellular machinery to carry out metabolic processes or produce energy. Instead, viruses must invade a host cell, hijack its machinery, and use the host's resources to replicate their genetic material and produce new viral particles. This dependence on a host cell defines viral biology and distinguishes them from other infectious agents that are capable of independent growth and replication outside of a host.

Bacteria can grow and divide on their own in various environments, while fungi can exist as single-celled organisms or multicellular structures and reproduce independently as well. Helminths, or parasitic worms, have complex life cycles that may involve multiple hosts, but they still possess the necessary systems to survive and grow outside of a host for certain stages of their life cycle. Thus, the unique requirement of viruses for a host to reproduce makes them the correct answer.