Earth Institute: The accelerating ebola epidemic in West Africa, which the World Health Organization (WHO) has called unprecedented, has so far killed more than half the 3,069 people who contracted the disease in Liberia, Sierra Leone, Guinea and Nigeria. The WHO fears it could eventually affect 20,000 people. Ebola can live in animals for years without making them sick; it is transmitted to humans through contact with an infected animal. Once in a human, the disease is spread by direct contact with the bodily...