What distinguishes a virus from a bacterium?

Viruses are fundamentally different from bacteria in that they cannot reproduce independently. Instead, they must infiltrate a host cell to replicate. Once a virus attaches to a host cell, it injects its genetic material, hijacking the cellular machinery of the host to produce new virus particles. This dependency on a host for replication is a defining characteristic of viruses.

In contrast, bacteria are single-celled organisms that are capable of living and reproducing independently in a variety of environments. They can carry out all necessary functions of life, including metabolism and self-reproduction. This key difference underlines the importance of understanding how viruses interact with living organisms, primarily highlighting their need for a host to multiply and persist.

The other options do not accurately characterize viruses or the distinctions between viruses and bacteria. For instance, viruses are not considered living organisms outside of a host, they lack a cell wall, and they cannot survive or replicate without a host.