Parasitic life cycles

Parasite life cycles can take a variety of forms, all involving the exploitation of one or 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 species have direct life cycles.

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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 and complete its life cycle 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. In intermediate hosts, parasites either do not reproduce or do so asexually.