Spirometra erinaceieuropaei is a tapeworm that infects domestic animals and humans. In humans infection is called sparganosis. The worm has an interesting lifecycle, the adult worm is present in the small intestine of cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s and dog

Dog

The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...

s where it may grow as long as 1.5 metres. Eggs from the worm are passed with the host feces, when they develop into a proceroid larva. This larva may be directly ingested by humans or may enter an intermediate host which include frogs, birds, snakes, rats and mice and become a plerocercoid larva. When cats, dogs, foxes or wolves eat the intermediate host the worm completes its life cycle becoming an egg producing adult. Because humans would normally ingest the worm at the proceroid stage the human is a dead-end host.