There are three main species of these trematode worms (flukes) --Schistosoma haematobium, S. japonicum, and S. mansoni -- that cause disease in humans. The larval forms of the parasite live in freshwater snails. The cercaria (form of the parasite) is liberated from the snail, burrows into skin, transforms to the schistosomulum stage, and migrates to the urinary tract (S. haematobium), liver or intestine (S. japonicum, S.mansoni) where the adult worms develop. Eggs are shed into the urinary tract or the intestine and hatch to form miracidia (yet another form of the parasite) which then infect snails, completing the life cycle of the parasite.
Adult schistosome worms can cause very serious tissue damage. Some schistosomes which cannot live within man nonetheless cause swimmer's itch.
Schistosomiasis is also called bilharzia after the short-lived German physician Theodor Bilharz (1825-1862).