Cotton Mather is often given the credit for inoculation during the small pox epidemic. Less often is it mentioned that inoculation was already popular in Africa and the Middle East. Even less often is it mentioned that Mather didn't come up with the idea himself, but got the idea from one of his slaves. A man named Onesimus (I wonder if he was named after the Biblical Onesimus?) told Mather about the practice, used in West Africa. Other slaves confirmed Onesimus' testimony, convincing Mather to try it on the general population. So, you can thank a black man and West African slaves for the fact that you've never had small pox if you were born and raised in the US.

References

1. Matthew Niederhuber. "The Fight Over Inoculation During the 1721 Boston Smallpox Epidemic". Harvard University: Science in the News. December 31, 2014
2. Arthur Boylston. "The origins of inoculation". Journal of the Royal Society of Medicine. July 2012