Schistosomiasis is a chronic infection. It is caused by parasitic worms called Schistosoma haematobium (Sh) that are spread by snails that live in rivers. It can lead to liver problems or bladder cancer. Praziquantel (PZQ) is a drug used to treat this infection. After taking it, some people develop increased resistance to reinfection with Sh. Some people with Sh infection can be infected with another worm called Mansonella perstans (Mp). Mp is spread through a biting insect called a midge. It rarely causes symptoms. However, researchers think that Mp infection could affect the body s response to PZQ treatment for or risk of reinfection with Sh.