Flat worms are largest organisms to parasitize human beings. They are also called flat worms because of their flattened bodies. All tape worms are parasites and require at least two hosts to complete their life cycle. Tape worms have a small head on which suckers and sometimes hooks are present to attach them to their host's intestine. Their length varies from few centimeters to 10 meters and their body is made of segments called proglottids. They are hermaphrodite i.e. Each proglottid has both sexes. The life cycle of tape worm involves two hosts. Primary host is the one in which the adult tape worm lives and reproduces. And secondary host is an animal, which is eaten by the primary host. For example; beef tape worm's primary host are humans and secondary host is cattle. If the sewage disposal conditions are not proper, tape worm eggs may be eaten by a cow while grazing. Because of the digestive juice of cow in the gut, larvae come out of the eggs which then find their way into the, muscular tissues. There they remain unchanged for several years. If the partially cooked cow meat of infested cow is eaten by humans the larvae are activated by digestive juices, where they manage to attach the intestinal wall with their suckers or hooks. There the larvae develop into adult tape worms. After fertilization sex organs disappear and are replaced by thousands of microscopic proglottids. At this stage proglottids drop from the tape worm and pass out of the host's body with faeces.