How Edward Jenner found vaccination?
Observation and deductive reasoning. Smallpox was a scourge
during Jenner's time, but he noticed that milkmaids contracted a
very mild illness similar to smallpox called "cowpox", from which
they recovered easily. He postulated that a tiny bit of the cowpox
serum could be injected into a well person, and the mild cowpox
might protect them from the deadly smallpox. It worked.