How did Edward Jenner discover that cowpox could protect against smallpox?

Edward Jenner, an English surgeon, discovered that cowpox could protect against smallpox by investigating common folk knowledge of the time: that dairymaids who contracted cowpox appeared to be immune to smallpox outbreaks.12

In 1796, Jenner tested this observation experimentally. He took material from a fresh cowpox lesion on the hand of a dairymaid named Sarah Nelmes and used it to inoculate an eight-year-old boy, James Phipps. After Phipps recovered from a mild illness, Jenner exposed him to smallpox matter. The boy did not contract the disease, demonstrating that he had acquired immunity.134

Jenner concluded that cowpox could be transmitted as a deliberate mechanism of protection, a procedure he termed vaccination—derived from the Latin word vacca, meaning "cow". This breakthrough provided a safer alternative to the existing practice of variolation, which carried significant risks of spreading the disease. Jenner’s findings were published in 1798, and his method eventually led to the global eradication of smallpox.1345