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.

There is nothing much to do, after all, in this area to the west of Abuja, Nigeria’s capital city, nor a better way to stay cool when the temperature soars.

But on a hot day in November, in the low-slung buildings of a government school in Gwagwalada, dozens of students told aid workers they had fevers, blood in their stool or urine and abdominal and body pains — the symptoms of an illness called schistosomiasis, or bilharzia.

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.

Gwagwalada more than meets the W.H.O. criteria. When children return to the river to swim, and adults turn to it as a water supply, “they cannot really avoid contact and reinfection,” said Amadou Garba Djirmay, who oversees the schistosomiasis program at the W.H.O.

“The main strategy is this treatment to really cure the majority of them,” he said.

Thought to affect 200 million people globally, schistosomiasis is considered one of the most important parasitic infections worldwide, second only to malaria. It has been a pervasive threat in Egypt and throughout the Nile Delta for centuries.