The parasites that cause schistosomiasis live in certain types of freshwater snails. The infectious form of the parasite, known as cercariae, emerge from the snail, hence contaminating water. You can become infected when your skin comes in contact with contaminated freshwater. Most human infections are caused by Schistosoma mansoni, S. haematobium, or S. japonicum.
Image: Left: Biomphalaria sp., the intermediate host for S. mansoni. Right: Bulinus sp., the intermediate host for S. haematobium and S. intercalatum. Center: Adults of S. mansoni. The thin female resides in the gynecophoral canal of the thicker male. Credit: DPDx