Which organism is described as an intracellular parasite that requires living host cells to replicate?

• A. Bacteria
• B. Toxins
• C. Fungi
• D. Viruses

Answer: (D)

Viruses are obligate intracellular parasites: they cannot replicate outside a living host cell because they lack their own metabolic and protein-making machinery. Their genome (DNA or RNA) is too simple to carry out transcription and translation on its own, so once a virus enters a cell, it hijacks the host’s ribosomes, enzymes, nucleotides, and energy to replicate its genome and produce viral proteins, assemble new virions, and then release them to infect other cells. This dependence on a living cell for replication is what set viruses apart from other organisms.

Bacteria are cellular and can often multiply independently in suitable conditions. Fungi are also cellular organisms that can grow and reproduce outside host cells, though some can live in hosts. Toxins are substances, not organisms, and do not replicate.