Parasites That Attack Animals and Man

SOME of the parasites that occur in animals also attack people. Some have to develop in an animal or in two different animals before they can invade human beings. Man then is the definitive, or final, host. He usually acquires the parasites by eating the intermediate animal hosts or (if they are small insects) by swallowing them accidentally. For some parasites, human beings are the intermediate hosts and animals are the final hosts. Even the few parasites that live only in man are related closely to others that live in animals. Farm animals, dogs and cats, and rats and mice contribute to man's burden of parasites. That burden tends to become lighter as our civilization advances, however, largely because the sanitary barriers that have come into existence impede the spread of parasites from parts of the world where standards of sanitation and hygiene are imperfect. Some of the major parasites that man shares with animals are discussed here. The examples illustrate that parasites can be acquired by eating infected food of animal origin and by being in close contact with animals that harbor the kinds of pests that can be transmitted to human beings.