Yes. He noticed how milkmaids caught cowpox, but didn't get smallpox. Cowpox was similar to smallpox but much milder so didn't kill people who caught it. The milkmaids who had caught cowpox didn't catch smallpox because their body had become immune to it after having cowpox. To test this theory, Jenner took some of the pus from a cowpox spot from a milkmaid and injected it into a boy named James Phipps. He fell ill, but didn't die. He then recovered and Jenner took the pus of a smallpox spot, in a high dose, and then injected this into James phipps. This was very risky because smallpox at the time was very deadly. James Phipps did not fall ill. The cowpox had made him immune to smallpox. This vaccination was done on more and more people to make more people immune to smallpox.
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Edward Jenner is considered responsible for the development of the smallpox vaccine. This vaccine eventually led to the world wide eradication of the disease.