MANY EPIDEMIC DISEASES that threaten the world today never infected humans on a large scale, or perhaps at all, until the late twentieth century. Some of these "emerging diseases" are caused by microorganisms that have only recently developed the power to infect humans or make them sick. Other emerging diseases have greatly increased the area over which they spread or the number of people they infect.

AIDS: collapsing defenses

AIDS is by far the best known of emerging diseases. Most scientists believe that this illness is caused by a virus called HIV (human immunodeficiency virus), which is very similar to viruses that cause AIDS-like illnesses in certain African monkeys and apes. Many AIDS experts suspect that an ancestor virus made the leap from these animals to humans when people in central or west Africa ate infected animals or perhaps were bitten or scratched. The...