Although many diseases such as measles, pneumonia, and pertussis (whooping cough) have probably inflicted humans for millennia, at least 30 new infectious diseases have appeared in the past two decades, In addition, many well-known diseases recently have reappeared in more virulent or drug-resistant forms. An emergent disease is one never known before or one that has been absent for at least 20 years. Ebola fever is a good example of an emergent disease. A kind of viral hemorrhagic fever, Ebola is extremely contagious and often kills up to 90% of those who are exposed to it. The disease was unknown until about 20 years ago, but is thought to have been present in monkeys or other primates. Killing and eating chimps, gorillas, and other primates is thought to be the route of infection in humans. AIDS is another disease that appears to have suddenly moved from other primates...