Looks at parasites--some benign, some deadly--that make humans their hosts. Examples are discussed in detail from the three major categories: single-celled; multicellular, or worms; and ectoparasites, those that live on the body as opposed to inside it. Experts show how these organisms proliferate through vectors, creatures that transmit parasites, such as the mosquito that carries malaria or the fly that transmits "river blindness." Interviews with victims and footage of parasitic damage are combined with commentary.