Which groups are commonly recognized as parasites?

• A. Bacteria and viruses
• B. Fungi and yeasts
• C. Prions only
• D. Protozoa, roundworms, and tapeworms

Answer: (D)

Parasites are organisms that live in or on a host and depend on that host for nourishment, often causing harm. The classic groups that fit this relationship are protozoa, along with helminths such as roundworms and tapeworms. Protozoa are single-celled organisms that can inhabit humans and cause illnesses like malaria or giardiasis. Roundworms and tapeworms are longer, multicellular worms that spend much of their life cycle inside a host, drawing nutrients from it.

Bacteria and viruses can cause disease, but they aren’t typically categorized as parasites in the same sense. Bacteria are cellular and can live independently in many environments, and viruses are non-living outside a host and are usually described as pathogens or infectious agents rather than parasites. Fungi and yeasts can also cause infections, but the strongest, most commonly recognized parasite examples are protozoa and the various helminths like roundworms and tapeworms. Prions are infectious proteins, not organisms that live as parasites, so they aren’t grouped with parasites.