relationship between two species of plants or animals in which one benefits at the expense of the other, sometimes without killing it. Parasitism is differentiated from parasitoidism, a relationship in which the host is always killed by the parasite; parasitoidism occurs in some Hymenoptera (ants, wasps, and bees), Diptera (flies), and a few Lepidoptera (butterflies and moths): the female lays her eggs in or on the host, upon which the larvae feed on hatching.

Parasites may be characterized as ectoparasites—including ticks, fleas, leeches, and lice—which live on the body surface of the host and do not themselves commonly cause disease in the host; or endoparasites, which may be either intercellular (inhabiting spaces in the host's body) or intracellular (inhabiting cells in the host's body). Intracellular parasites—such as bacteria or viruses—often rely on a third organism, known as the carrier, or vector, to transmit them to the host. Malaria, which is caused by a protozoan of the genus Plasmodium transmitted to humans by the bite of an anopheline mosquito, is an example of this type of interaction. The plant disease known as Dutch elm disease (caused by the fungus Ceratocystis ulmi) can be spread by the European elm bark beetle.

A form of parasitism called brood parasitism is practiced by the cuckoo and the cowbird, which do not build nests of their own but deposit their eggs in the nests of other species and abandon them there. Though the cowbird's parasitism does not necessarily harm its host's brood, the cuckoo may remove one or more host eggs to avoid detection, and the young cuckoo may heave the host's eggs and nestlings from the nest.

Another form of parasitism, such as that practiced by some ants on ants of other species, is known as social parasitism. Parasites may also become parasitized; such a relationship, known as hyperparasitism, may be exemplified by a protozoan (the hyperparasite) living in the digestive tract of a flea living on a dog.