The larval stages of three tapeworms of the genus Echinococcus can cause severe disease in humans. All three normally become adults in the intestines of dogs or other canids. Eggs are passed in the feces and, if ingested by a herbivore, develop in the liver or other organs into a saclike container of larvae, the hydatid cyst. Carnivores become infected by eating cysts with the flesh of the herbivore. Echinococcus granulosus, which commonly has a sheep–dog cycle, but which may also infect goats, cattle, swine, and camels, is the most likely to infect human beings. Human echinococcosis occurs primarily in sheeprearing areas. Dogs ingest cysts in the offal of dead sheep and pass eggs in their feces. Humans acquire the eggs from a dog’s fur or from contaminated food or water. Cysts holding 2 or more liters of fluid and larvae can grow for years in the liver, lungs, brain, or other organs and exert enough mechanical pressure to cause grave or fatal consequences. Rupture of a cyst by trauma or surgery releases daughter cysts, which may grow elsewhere in the victim; the hydatid fluid can cause fatal anaphylactic shock. Hydatid cysts in humans and animals have been known since Roman times, but, as was true for the other tapeworms, the relationship between the larval cyst and the adult worm was not suspected until the eighteenth century. E. granulosus was described as a separate species in 1850, and its life cycle was worked out with feeding experiments in 1863.