OGI, Nigeria — Whatever secrets the turgid brown depths of the Sacred Pond of Ogi may keep, there is one they betray quite easily: why it is so infuriatingly hard to wipe even one disease off the face of the earth. Ogi is one of the last areas of Nigeria infested with Guinea worm, a plague so ancient that it is found in Egyptian mummies and is thought to be the "fiery serpent" described in the Old Testament as torturing the Israelites in the desert. For untold generations here, yardlong, spaghetti-thin worms erupted from the legs or feet — or...