An association in which one organism (the parasite) lives on (ectoparasitism) or in (endoparasitism) the body of another (the host), from which it obtains its nutrients. Some parasites inflict comparatively little damage on their host, but many cause characteristic diseases (these are, however, never immediately fatal, as killing the host would destroy the parasite’s source of food; compare parasitoid). Parasites are usually highly specialized for their way of life, which may involve one host or several (if the life cycle requires it). They typically produce vast numbers of eggs, very few of which survive to find their way to another suitable host. Obligate parasites can only survive and reproduce as parasites; facultative parasites can also live as saprotrophs. The parasites of humans include fleas and lice (which are ectoparasites), various bacteria, protists, and fungi (endoparasites causing characteristic diseases), and tapeworms (e.g. Taenia solium, which lives in the gut). See also hemiparasite.

https://www.cdc.gov/DPDx/ Descriptions of human parasites feature in this website from the US Centers for Disease Control and Prevention