Pestilence is the most terrifying of all plagues, in part because it is invisible to the human eye. The viruses and bacteria that cause infectious diseases are far too tiny to be seen, which means that they can strike anywhere, anytime, without being noticed. Even worse, they attack from within the body, bringing a foul death that strips the sufferer of dignity and humanity. The survivors suffer, too: They must endure not only grief at the passing of their loved ones, but also the fear that they will be next.

Most infectious diseases do not present a major threat to hu- mankind. Some simply are not very dangerous. A person is likely to survive dozens of colds during the course of a lifetime as well as a handful of more serious ailments such as influenza, chicken pox, and strep throat. Other diseases, such as Ebola hemorrhagic fever, are...