The word "plague" means different things to different people. To a medical professional, it conjures visions of gruesome diseases. To a farmer, it may represent disastrous crop blights that lead to financial ruin and, perhaps, starvation. To a govern- ment agency, it may mean plant and animal invaders intent on destroying a country's natural resources. And to residents of rural Africa and other insect-ravaged parts of the world, the word may signify an especially elemental horror: clouds of crawling, chewing locusts that descend thickly and without warning from a clear blue sky, devour every scrap of vegetation they can find, and then move on, leaving barren fields in their wake.

No matter what form it takes, a plague by its very definition is calamitous, generally bringing death, famine, or economic destruction. Under the most favorable circumstances, a plague...