For the children of Gwagwalada, the river that runs alongside their town is a choice destination. Never mind that their parents forbid them to play in the water, never mind that the water is infested with tiny flatworms that take up residence in their intestines. The students who reported symptoms, and those who did not, all received doses of a drug called praziquantel that treats the illness. This strategy — called mass drug administration — is endorsed by the World Health Organization for any region where more than one in five residents has schistosomiasis. Thought to affect 200 million people globally, schistosomiasis is considered one of the most important parasitic infections worldwide, second only to malaria.