British Dr. Edward Jenner successfully demonstrates that inoculation with cowpox matter provides immunity against smallpox, marking the beginning of modern vaccination. He famously vaccinated James Phipps, a boy of age 8, with cowpox, and, then later tested his immunity by exposing him to smallpox. James Phipps did not contract smallpox, proving Jenner's discovery. More
Painting by Ernest Board (1877–1934). Image in the Public domain via Wikimedia