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He had noted that milkers of cows who contracted “cowpox” (a milder form of the disease) were said to have acquired immunity to smallpox. Jenner decided to test a vaccine on an eight-year-old boy, who used “cowpox” samples from milkmaids. After exposing the boy to smallpox and showing no symptoms of the deadly disease, Jenner realized he had developed a way to stop the disease.

While Jenner’s experiment was unethical by today’s standards, it became a very important discovery. Smallpox was fatal in humans in about 30% of cases at the time.

In North and South America, this deadly disease had killed large numbers of natives after European settlers introduced smallpox and other new diseases to those continents. After Jenner’s vaccine was created, it was first used by Spain to inoculate people across the empire. Later, British and US residents also began to be vaccinated.

“In the mid-20th century, right after World War II, the world thought, ‘Why don’t we get rid of smallpox?'” says Nahera. “And so they made an effort like never before.” campaign that led to smallpox had already been eradicated in 1979.