Viruses are best described as which of the following?

• A. Cellular organisms that infect bacteria only
• B. Acellular infectious agents that can infect cells
• C. Photosynthetic plants
• D. Multicellular parasites

Answer: (B)

The important idea is that viruses are acellular infectious agents that can infect cells. They lack cellular structure and do not carry out metabolic processes on their own, so they cannot reproduce without a host. A virus particle contains nucleic acid (DNA or RNA) wrapped in a protein coat, and sometimes a lipid envelope, but no full cellular machinery. To replicate, viruses hijack the host cell’s replication and protein-making systems, which is why they depend on living cells. This explains why they’re not cellular organisms and not limited to bacteria—there are viruses that infect plants and animals too. They’re not photosynthetic plants or multicellular parasites, which require their own cellular organization and tissues.