For thousands of years, smallpox had been one of the world's biggest killers. From the terrible plague that nearly wiped out the entire Aztec and Inca populations in the 1500s to the outbreak in the colonies that killed many of George Washington's troops during the Revolutionary War, smallpox has been responsible for some of the worst epidemics of all time. In the 1960s, the World Health Organization started a worldwide vaccination campaign in an...