The first case of smallpox in England was recorded in 1514 and it spread quickly. As many as 10% of the population contracted the disease each year, and up to 30% died from it. At the time, there was no cure for smallpox—the only way to prevent contracting it was to stay away from people who had it.
In 1796, a physician named Edward Jenner noticed that milkmaids who contracted cowpox were immune to smallpox. Cowpox is a milder form of smallpox that doesn’t always cause symptoms but can sometimes lead to a blister on the hand or arm where it entered through contact with an infected cow’s udder or milk pail.
Jenner decided to test whether or not injecting someone with cowpox would protect them from getting smallpox. He injected a boy with cowpox, waited until he developed a blister, then exposed him to smallpox two weeks later. The boy didn’t get sick! This meant that he could be vaccinated against smallpox without having to go through the dangerous process of getting infected with it first—a process which often resulted in death even if you survived infection
