Which category of pathogens is known for being unable to reproduce outside of living host cells?

Viruses are categorized as pathogens that fundamentally rely on living host cells for reproduction. Unlike bacteria, fungi, and protozoa, which can survive and reproduce independently in various environments, viruses lack the necessary cellular machinery for metabolic processes and replication. They must invade a host cell and hijack the host's cellular processes to replicate their genetic material and produce new virus particles. This dependency on host cells distinguishes viruses from other categories of pathogens, making them unique in their reproductive strategy and lifecycle. In summary, the correct answer confirms that viruses cannot reproduce outside of living cells, which is a defining characteristic of these pathogens.