Illustration of a fluke of the genus Schistosoma, which is a parasitic worm that causes schistosomiasis (also known as bilharzia, bilharziosis or snail fever). Cercariae released by infected snails enter the water and penetrate the skin of a human. They become schistosomulae and migrate to the liver, where they mature. Paired adult worms migrate to the bowels and lay eggs, which are shed in the stool and hatch, releasing miracidia, which penetrate snail tissue. The snails then release cercariae into the water and the cycle is repeated.