What best defines a parasite?

• A. An organism that always harms its host or causes disease.
• B. A virus that infects host cells.
• C. A mutualistic organism that provides nutrients to the host.
• D. An organism that lives on or in another organism (its host) and benefits by deriving nutrients at the host's expense.

Answer: (D)

Parasites are organisms that live on or inside another organism (the host) and gain nutrients from the host, often at the host’s expense. This definition covers a wide range of relationships, from external parasites like ticks and lice to internal ones like tapeworms and malaria parasites, where the parasite relies on the host for survival and resources. Saying a parasite always harms the host is too absolute, and focusing only on viruses narrows the concept to a specific type of organism. A mutualistic partner, on the other hand, provides benefits to the host, which is the opposite of what a parasite does. The key idea is the dependence on the host and nutrient extraction at the host’s expense, which is precisely captured here.