High in the mountains of China's Sichuan Province, George Davis of George Washington University is collecting tiny, seemingly harmless snails from a muddy ditch that runs through terraced fields. As the evolutionary biologist wields his tweezers, he contemplates how massive disruptions of the environment produce widespread problems, including emerging epidemics. "Diseases like SARS can pop up because of the interference of man in the ecosystem," he explains. "And now we have one of the most dynamic ecosystem changes in history in the Three Gorges Dam."

His concern: infections by parasitic worms, producing the fast-spreading and potentially deadly schistosomiasis that infects his collected snails--and humans. Davis and his colleagues from China's Institute for Parasitic Diseases (IPD) in Shanghai are finding that the Three Gorges Dam--the world's largest--is driving an increase in schistosomiasis infection densities around China's greatest lake.