As far as humans are concerned, plagues are disastrous. Epidemics and famines have killed untold millions of people; swarms and blights have driven countless more into ruin. Families are torn apart, industries collapse, and economies fail under the plague's assault. The natural perspective, however, is very different from the human one. From nature's viewpoint, a disease that wipes out 25 million people is no disaster. It is simply an indication that some imbalance has occurred, allowing the disease-causing microorganism to run rampant. Disease outbreaks are natural processes in- tended to restore balance, as are swarms, bioinvasions, and blights.

Although people have always fought these natural processes to the best of their ability, only in recent centuries have they enjoyed significant success. Humankind now knows how to overcome many dangerous diseases, bring some swarms under control, and increase crop security worldwide, to...