The guinea worm is set to go the way of smallpox, becoming the second human disease to be fully wiped out—thanks in part to work by Jimmy Carter's Carter Center. "We are approaching the demise of the last guinea worm who will ever live on Earth," says Carter himself. Infections, some 97% of which take hold in South Sudan, have been reduced by 99% since work began in 1986. There were 3.5 million cases then; the first few months of 2012 have seen just five.
The guinea worm parasite is transmitted solely through drinking water (it's the only disease where that's the case), Scientific American notes. And it's a pretty gnarly experience: After people consume the parasite's larvae, the roundworm develops inside their bodies over the course of a year. Then it exits by way of a leg or foot in a process that causes awful pain—which some try to soothe by keeping the limb underwater, which only helps foster the disease. The Carter Center and other groups have used water filters and education, as well as a larvicide, to fight the disease. (Read more guinea worm stories.)