The Carter Center has marked another step along the road to eradicating the misery-causing Guinea worm. On Thursday the center, which has led the eradication campaign since 1986, announced that Ghana has conquered the disease. That leaves only Ethiopia, Mali and South Sudan on the active list. The worm's larvae infect people who drink contaminated water; a year later, a 40-inch long worm exits through the skin. More than 3.5 million cases of the disease were reported worldwide when the campaign began; this year, about 800 cases were reported through June.