Who is Edward Jenner? What is he credited with doing? How did he achieve this?

Jenner was an English country doctor, and the first to administer the smallpox vaccine. He noticed that dairymaids who'd been infected by cowpox seemed to be immune to smallpox. His initial experiment involved administering cowpox pus to a young boy's arm, a few months later, he did the same with pus derived from a smallpox victim. The boy did not catch the disease, and Jenner's "challenge trial" was successful. The discovery and naming of this vaccination took place in 1796.