He’s a guy that may have saved more lives than any other single person in history. In the 1790’s, he noticed that “milkmaids” seldom came down with smallpox. He developed a theory that the blisters which “milkmaids” commonly had as part of their work (from a disease called cowpox) somehow protected them from smallpox. In 1796, he tested his theory by inoculating the 8 year old son of his gardener with cowpox blisters.
Later, he conducted what at the time was an accepted practice called “variolation”, meaning he purposely infected the boy with a small amount of smallpox in a controlled manner that was commonly practiced at the time to “immunize” people against further infection with smallpox. “Variolation” produced lasting immunity to smallpox because the person was infected with the smallpox virus. But, because variolation had a much lower mortality rate (about 1%) than the disease itself (20–30%), it was considered an acceptable practice at the time. Since the boy never got sick from repeated exposures, Jenner concluded that cowpox could be made into an effective smallpox vaccine.
Using this information, public health proceeded to develop what became the first smallpox vaccine using the cowpox virus (the word vaccine comes from the root latin word “vacca” which means cow). Vaccination to prevent smallpox was soon practiced all over the world. The cowpox virus was later replaced by a different version of the smallpox virus. Public health efforts eradicated smallpox from the globe in the late 1970s. The last naturally occurring case was in Somalia in 1977 (followed by a couple of lab accident cases in 1978).