Which non-living infectious agent can replicate only inside living cells?

• A. Bacteria
• B. Virus
• C. Fungi
• D. Prion

Answer: (B)

Viruses are obligate intracellular parasites: they cannot carry out metabolism or reproduce on their own outside a living cell. To replicate, a virus must enter a host cell and hijack the cell’s machinery to synthesize viral components and assemble new virions. Because they rely entirely on a living cell to reproduce, they are considered non-living infectious agents that replicate only inside living cells. Bacteria and fungi are living organisms that can grow and reproduce independently, while prions are infectious proteins that propagate by inducing misfolding of normal proteins rather than using cellular genome replication. This distinction makes viruses the type that fits the description.