What is a parasite?

A parasite is specifically defined as an organism that lives on or inside another organism (the host) and derives nutrients at the host's expense. This relationship typically harms the host, as the parasite benefits while drawing resources or energy from it. Examples of parasites include tapeworms, fleas, and lice, all of which rely on their hosts for survival and reproduction while causing varying degrees of harm.

The other choices do not reflect the definition of a parasite accurately. An organism that is beneficial to its host describes a mutualistic relationship rather than a parasitic one. A microorganism that causes food decay refers to decomposers, which play a role in nutrient recycling but are not parasites in a living host context. Lastly, a virus that infects bacteria is known as a bacteriophage, which has a different biological interaction than that of a parasite with its host. These distinctions are important in understanding the various relationships organisms have with each other.