Vaccinations
Edward Jenner was a keen observer throughout his life. Many of his patients held the belief that people who had been affected by cowpox were immune to the effects of small pox. Milkmaids at the time often caught cowpox. This caused a few days of discomfort and some small pocks. In 1796 one of his patients came to him, and he diagnosed her as having cowpox, which she caught from one of the cows she recently came in contact with. Jenner decided to use this opportunity to test his theory, by giving it to someone who not yet been exposed to smallpox. His test patient was an 8-year-old boy who was the son of his gardener. He rubbed puss from the milkmaid pocks into scratches in the boys arm. As expected the boy became mildly ill and within a few days returned to normal. Then came the ultimate test he exposed the box to small pox, a process called variolation. To the relief of everyone, the boy was immune to smallpox.
To further prove his theory Jenner repeated the experiment. Finally in 1798 he published his results. He continued to submit and publish results for the next two years. The medical community at the time didn't believe the findings. Many tried to replicate the experiments but failed. It was later proven that the cow pos samples were cross-contaminated with the small pox samples, which invalidated the vaccine. Eventually the medical community accepted the evidence and began vaccinations on large scale. As a result deaths from smallpox plummeted.