Parasitism is a type of adaptation. A parasite always needs a host to feed itself to its costs. Those parasites that live outside of the body of the host, for example, lices and carrapatos, are external parasites, or ectoparasitas. While the ones that live inside of the body of hospediro are the internal or endoaparasitas parasites. Worms are parasites and go to speak on two illnesses caused for them: Esquistossomose: The individual contaminated for the Schistossoma Mansoni, eliminates in excrements hundreds of eggs, that give to origin the small larvae, called miracdios. In an aquatic environment, a miracdio can contaminate one caramujo. After the miracdio if to reproduce inside of caramujo, leaves it small called larvae cercrias. The cercrias can infectar one another human being that drinks or makees contact with water where they exist cercrias. Tenase the worms known parasites as tnias are the causers of tenase. Two species of tnias that attack the human being and parasitam its intestine are the Taenia Solium and the Taenia Saginata. The pig is intermediate host, in case that the human being exerts this paper, it acquires Cisticercose.