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Definition: The larger of two organisms in a symbiotic relationship. An organism or cell on or in which a parasite lives or feeds. A definitive host is an organism in which a parasite reaches sexual maturity. The anopheles mosquito is the definitive host for the malaria plasmodium because, while the mosquito is not adversely affected by the plasmodium's presence, it is the organism in which the plasmodium matures and reproduces. An intermediate host is an organism in which a parasite develops but does not attain sexual maturity. Humans and certain other vertebrates are the intermediate host of the malaria plasmodium. A paratenic host is an organism which may be required for the completion of a parasite's life cycle but in which no development of the parasite occurs. The unhatched eggs of nematodes are sometimes carried in a paratenic host such as a bird or rodent. When a predator eats the paratenic host, the eggs are ingested as well.
Source: The American Heritage Science Dictionary. Boston: Houghton Mifflin, 2005. Credo Reference. Web. 10 August 2012.