LAKE VICTORIA looks an inviting place for a quick dip. Don’t. Its waters are replete with tiny parasitic worms, called schistosomes, that drill through human skin and damage organs ranging from the liver to the brain. The result is bilharzia, an illness that affects about 200m people, particularly in Africa. It kills more than 10,000 of them a year, but that is not its only serious consequence. Many victims are children, and the parasites stunt both their physical growth and their cognitive abilities, damaging their personal prospects and dragging down the economies of what are already poor countries.