A disease in which humans are parasitized by any of three species of blood flukes: Schistosoma mansoni, S. haematobium, and S. japonicum. Adult S. mansoni prefer the veins of the hemorrhoidal plexus, S. haematobium those of the vesical plexus, and S. japonicum those of the small intestine. The disease is also known as bilharziasis.

An embryonated egg passed in feces or urine hatches in fresh water, liberating a miracidium larva which penetrates into specific gastropod snails. The larval cycle in the snail lasts for about 1 month. The cercaria emerges from the mollusk, swims in the water, and penetrates the skin of the final host upon coming in contact with it.

Schistosomiasis is an agricultural hazard for all ages in irrigated lands or swamps. Elsewhere fluvial waters are the main source of infection, in which case incidence is marked in human beings who are less than 15 years old and is higher among boys than among girls.