How did Edward Jenner find the vaccine for smallpox?

Edward Jenner discovered the smallpox vaccine in 1796 when he observed that milkmaids who had contracted cowpox, a less severe disease, seemed immune to smallpox. He hypothesized that exposure to cowpox could protect against smallpox. To test his theory, he inoculated a boy named James Phipps with material taken from a cowpox sore, and later exposed him to smallpox, finding that he did not become ill. This pioneering work laid the foundation for the development of vaccines and the eventual eradication of smallpox.