Scientists have long known that in the tropics, dams increase the number of people getting a vicious disease called snail fever. But it is less clear why this happens. We identified the key players in this mystery and put together different geographical, ecological and epidemiological maps to figure it out. It turns out that dams limit the migrations of river prawns, which are important predators of the snails that host the parasites that make people sick. With fewer prawns, there are more snails, and thus more parasites infecting more people. We estimate that almost 400 million people are affected by this ecological and technological chain reaction. Our results suggest that the restoration of river prawns can be an effective tool for decreasing snail fever worldwide.