What best defines viruses?

• A. Intracellular parasites that require living host cells to replicate.
• B. Unicellular organisms that cause disease by toxins.
• C. Non-living infectious particles that do not require hosts.
• D. Organisms that always cause disease.

Answer: (A)

Viruses are intracellular parasites that require living host cells to replicate. They can’t carry out metabolism or reproduce on their own; instead, they inject their genetic material into a host cell and hijack that cell’s machinery to make new viral components, which then assemble into new virus particles. Outside a cell, viruses are inert particles and are often described as non-living because they lack independent metabolic activity. This dependence on a host for replication is the defining feature, which is why this description fits best.

The other statements don’t fit because viruses aren’t unicellular organisms, they don’t reproduce by toxin production like some bacteria, and they don’t always cause disease—many infections are asymptomatic or are controlled by the immune system. They also cannot replicate without a host, so saying they don’t require hosts is inaccurate.