One thing to add is that Edward Jenner learned the principle of vaccination from a man he kept as a slave. He asked if smallpox existed in the place the man had been born. The man showed him a slice mark on his arm and described how when a person developed smallpox, someone who slice one of the poxes, and then slice the healthy children to effectively inoculate them. That's the idea that Jenner is credited with, but was practice around the world, it turns out, just not in European nations.