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Examples: A cell can be host to a virus, a legume plant can be host to helpful nitrogen-fixing bacteria, an animal can be host to a parasitic worm, e.g., a nematode.

A primary host or definitive host is a host in which the parasite grows mature; a secondary host or intermediate host is a host that harbors the parasite only for a short transition period. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host.