Which statement best describes parasites?

Which statement best describes parasites?

• A. Organisms that cause disease in hosts
• B. Organisms that live independently and never interact with hosts
• C. Organisms that live in water and feed on plankton
• D. Organisms that live on or in a host organism and benefit at the host's expense

Answer: (D)

Parasites are organisms that live on or inside another organism and gain benefits at the host's expense. This means the parasite relies on the host for nutrients, a habitat, or a means of reproduction, and the host is harmed or at least negatively affected in some way. It’s not necessary for every parasite to cause disease, though many do; the essential idea is the dependent, benefit-for-self at the host’s cost, rather than a free-living lifestyle. The alternative describes an organism that lives independently with no interaction with a host, which isn’t parasitic. Another alternative describes a free-living aquatic organism that feeds on plankton, also not parasitic since it doesn’t rely on a host. So the statement that best captures parasitism is that parasites live on or in a host and benefit at the host’s expense, encompassing both external and internal parasitic relationships. Examples include ticks on animals or tapeworms in the intestines, illustrating the host-dependent, resource-stealing aspect.