Imagine a swamp. You can smell it before you see it. Flies dance on the surface of a green, stagnant film, congregating with the animals that come to drink from it several times a day. The water is still and dark, cloaking the debris, waste, and bacteria that lie hidden underneath. This is where Moris, a poor peasant farmer in Uganda, collects water for his family. As he fills several jerry cans with the gray, slimy liquid, he thinks of what brought him to the swamp that day. During the dry season in Eastern Africa, most water sources have...