What is a parasite?

• A. A bacterium
• B. Organism living off a host
• C. A plant
• D. A virus

Answer: (B)

Parasites are organisms that depend on a host for resources, living on or inside the host and gaining benefits at the host’s expense. In livestock, this includes worms in the gut or external pests like ticks and lice. The defining idea is the relationship where the parasite uses the host for nutrients or habitat, typically harming the host in the process. Among the options, the description of an organism living off a host best captures this relationship. A bacterium is a type of organism, but not all bacteria are parasites and many live independently. A plant describes a plant, not the parasite lifestyle. A virus relies on host cells but isn’t an organism in the traditional sense, and the common, straightforward definition of a parasite emphasizes the host-dependent relationship.