What type of pathogen cannot reproduce without a host?

Viruses are unique among pathogens in that they cannot reproduce independently; they require a host cell to replicate and propagate. Unlike bacteria, which can divide and grow on their own in suitable environments, viruses consist of genetic material enclosed within a protein coat and are entirely dependent on living cells for their life cycle. When a virus infects a host cell, it hijacks the cellular machinery to produce new virus particles, ultimately leading to cell death or dysfunction in the process. This crucial dependency on a host for reproduction distinguishes viruses from other types of pathogens, such as bacteria, parasites, and fungi, which can survive and proliferate outside a host under certain conditions.