How are parasites defined?

• A. Microorganisms that derive nutrients from dead organic material
• B. Organisms that derive nutrients from living hosts
• C. Microorganisms that cause fermentation
• D. Viruses that infect bacteria

Answer: (B)

Parasites are organisms that obtain nutrients from living hosts, relying on the host’s resources while living on or inside it. This captures the idea of deriving sustenance from a living organism, which is what makes them distinct from other microorganisms.

The other concepts describe different roles: organisms that feed on dead organic material are decomposers, not parasites; those involved in fermentation are about transforming substrates, not living off a host; and while some viruses depend on host cells to replicate, the general notion of a parasite is centered on deriving nutrients from a living host, not just being a virus that infects a specific organism.