Schistosomiasis, also known as Bilharzia, is an infection caused by a parasitic worm that lives in fresh water in subtropical and tropical regions. Schistosomiasis is second only to malaria as the most devastating parasitic disease. The parasites that cause schistosomiasis live in certain types of freshwater snails. The infectious form, known as cercariae, emerge from the snail and then contaminate the water. People become infected when their skin comes into contact with the contaminated freshwater. Most human infections are caused by Schistosoma mansoni, Schistosoma haematobium, or Schistosoma japonicum.