History of Microbiology Practice Exam

Viruses are not independent, cellular organisms. They lack cellular structures and metabolic systems, so they cannot carry out replication on their own. Instead, they must hijack a living cell’s machinery—enzymes, ribosomes, and energy pathways—to replicate their genetic material and assemble new virus particles. This makes them acellular obligate parasites: they require a host cell to reproduce at all.

That’s why the statement that viruses are acellular obligate parasites requiring a host cell to replicate is the best description. The other ideas don’t fit: viruses do not replicate without a host, they are not cellular organisms with a nucleus, and while many viruses are smaller than bacteria (with a few exceptions), the general point is that viruses are much smaller and fundamentally different from bacterial cells.