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A parasite is defined as an organism that depends upon a living host for one or more of its essential metabolic requirements. This definition does not imply that the relationship necessarily damages the host, and occasionally two organisms benefit mutually from a close association, one extreme within a wide range of relationships. Many terms have been used to describe the relationships between parasites and their hosts; but few can be given an exact or exclusive definition. It is not, for example, possible to draw a sharp line between so-called endoparasites living within the body, which, in the case of some intestinal worms, may not invade the tissues, and ectoparasites living on the body surface, which, like the scabies mite, may in fact penetrate the epidermis. Similarly, when more than one species of host is successively infected during the life history of a parasite, they are not described in consistent terms. The definitive or final host is usually designated as the one in which the parasite reaches sexual maturity, and the intermediate host is one in which it undergoes larval development or an asexual phase. Thus, the human is definitive host for the so-called pork tapeworm Taenia solium , but may also act as an intermediate host in harbouring the bladder worm or cysticercus stage, which normally occurs in the pig. On the other hand, the human—as well as the sheep, the cow ... log in or subscribe to read full text

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