Human existence has always been overshadowed by deadly infectious diseases. For thousands of years before the advent of vaccines, epidemics of smallpox, bubonic plague, influenza, and numerous other kinds of contagion wiped out millions of people and caused tremendous suffering and misery. Throughout most of these long ages, people did not even realize what was killing them. The germ theory-the concept that germs cause disease-was unknown until the nineteenth century. Likewise, the development and administration of life- saving vaccines did not begin until that same century.

Yet long before vaccination became a reality, people in some parts of the world recognized the basic principle behind a vaccine-using physical samples of a disease to fight a disease, thereby giving the body immunity, the ability to resist the sickness. In one way or another, members of various past cultures stumbled on and utilized this...