Flow diagram demonstrating the roles of human water contact, and immunological and physiological factors, in determining schistosome infections. Point 1 demonstrates that since water from safe sources should be free of cercariae, provision of such water should prevent schistosome infections. However, as shown at point 2, the provision of safe water often does not prevent all contact with infested water. Point 3 shows another barrier to schistosome infections, namely the host’s immune system and physiology, which may kill invading cercariae before they can develop into adult schistosomes and cause pathology. Despite the host’s immunological and physiological defences, some cercariae successfully develop into adult worms (point 4). The relative importance of the water contact versus immunology and physiology, in preventing schistosome infections, is poorly understood.