The first signs of illness in Suleiman Djarra appeared during a heavy rainstorm a few years ago. The 2-year-old suddenly stopped eating and then developed severe diarrhea, which continued for days, draining him of energy. On the third day, Suleiman's mother Aiseta Traoré carried his listless body to the road outside their village in southern Mali and hitchhiked to the nearest hospital, about 9 miles away. There, she says, a doctor gave her a pack of vitamins and advised her to take the boy home to recover. Hours after Traoré and Suleiman reached their village, though, the boy died.
Generations of...