In 1986, after almost five years of construction, the Diama Dam was finally completed along the mouth of the Senegal River. The dam stopped saltwater from intruding upstream, thus creating a stable reservoir of freshwater for farmers and for Senegal’s capital city of Dakar.

But it also had unintended consequences. By restraining the saltwater, the dam favoured the growth of freshwater algae and plants, which in turn fed large numbers of snails. The snails are hosts for parasitic flatworms that cause schistosomiasis—a horrible water-borne disease that damages the kidneys, bladder, intestines, and liver. As the snail population boomed, they triggered a huge outbreak of schistosomiasis, which spread with unprecedented speed and still persists today. In some places, more than