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A parasite host is an organism on or in which the parasite lives and from which it derives its nutrition. The host is usually not related taxonomically to the parasite although this is not always the case (see intra‐specific parasites). Most parasites are highly host specific and only infect one host species or a group of closely related species. This is because all hosts represent a unique challenge in terms of the complex adaptations the parasite requires to evolve to identify, invade, and survive within/upon them. Nevertheless, a few parasite species, exploit a wide range of hosts. For example, the protozoan parasite Toxoplasma gondii infects, grows, and asexually reproduces in virtually all warm‐blooded vertebrates although sexual reproduction only takes place within the small intestine of cats.

Hosts can be divided into classes, depending upon the role they play in the parasite’s life cycle. The ‘definitive’ (or final) host is the one in, or on, which the parasite reaches maturity and undergoes sexual reproduction, whilst the ‘intermediate’ host is the one in which the parasite undergoes its developmental stage(s). There may be just one or several intermediate hosts and the parasite may or may not undergo asexual reproduction during this time, but it cannot develop into an adult or reproduce sexually. In this way, some parasites exploit their hosts to maximum effect by combining the reproductive power of asexual reproduction in the larval stage with the advantages of sexual reproduction during the adult stage.

Parasites devote more of their energies to reproduction than free‐living animals because they do not have to worry about food, shelter, and fluctuations in environmental conditions. This is important because the chances of any offspring locating and establishing themselves within a suitable host are very low. The completion of a parasite’s life cycle sometimes depends upon the death of the intermediate host and the subsequent consumption of the larval form by the definitive host. In this situation, the parasite is often very pathogenic in its intermediate host but has relatively minor effects on the definitive host. The intermediate host is not always killed or consumed by the definitive host. For example, after undergoing asexual reproduction in the snail intermediate host, the cercariae of the liver fluke Fasciola hepatica physically and chemically bore their way out and swim off to transform into metacercariae attached to aquatic vegetation. The snail survives the damage to its tissues, and the lifecycle is completed when the metacercaria are consumed by the sheep definitive host (see Section 5.2.1.1.1 for more details).