Which statement best defines parasitism?

• A. Both organisms benefit
• B. One organism benefits and the other one is harmed
• C. The other one is harmed
• D. Neither affected

Answer: (B)

Parasitism is a relationship where one organism benefits by taking resources from another, while the other organism is harmed as a result. The parasite gains nutrients, habitat, or a means of transport, often living on or inside the host for an extended period. The host, in turn, bears a cost—reduced growth, reproduction, or survival—though the parasite typically does not kill it quickly. This differs from mutualism, where both partners benefit, and from commensalism, where the host is largely unaffected. Examples include tapeworms living in a human intestine or fleas feeding on a dog. Therefore, the statement that one organism benefits and the other is harmed best defines parasitism.