The History of Vaccines
Long before anyone understood what a virus was, people noticed that surviving certain diseases you against catching them twice. In eighteenth-century England, Edward Jenner acted on the observation that milkmaids who had caught cowpox seemed to smallpox. His experiment was crude by modern standards but the principle was . A century later Pasteur showed that the agent could be weakened deliberately in the laboratory. The twentieth century then the technique to polio, measles and dozens of other diseases. Smallpox itself was worldwide in 1980, the only human disease so far to have been entirely.
