By the 1790s when English physician Edward Jenner (right) was treating patients, people all over the world had been trying to protect themselves against the scourge of smallpox for centuries. Jenner observed that people who caught cowpox, a similar but less virulent disease, never got smallpox, and this led him to wonder if something about cowpox was protective. To test his hypothesis, he injected a young boy with material from cowpox scabs. After the boy’s bout of cowpox was over, Jenner injected him with pus from a smallpox sore. The boy stayed healthy, and the episode led to the discovery of vaccination—a term that literally means “encowment.” What do you think would happen if a physician tried this experiment today?