Parasite life cycles can take a variety of forms, all involving the exploitation of one more hosts. Those that must infect more than one host species to complete their life cycles are said to have complex life cycles, while those that infect a single single species have direct life cycles. A parasite is an organism that lives in or on the living tissue of a host organism at the expense of it. ... In biology, a species is the basic unit of biodiversity. ...

If a parasite has to infect a given host in order to complete its life cycle, then it is said to be an obligate parasite of that host; sometimes, infection is facultative -- the parasite can survive without infecting that particular host species. Parasites sometimes infect hosts in which they cannot complete their life cycles; these are accidental hosts.

A host in which parasites have sexual reproduction is known as the definitive, final or primary host, while other hosts in the life cycle known as intermediate hosts.