Cotton Mather was a Puritan minister and amateur scientist who lived in Boston in the early 1700s. When a smallpox outbreak struck the city, a man named Onesimus whom Mather had enslaved recommended an inoculation technique that he’d seen used in his homeland of West Africa. Using Onesimus’s advice, Mather helped other scientists develop a method of inoculation that saved many lives and formed the foundation of our modern-day vaccination techniques. Yet he never gave Onesimus any credit and never freed Onesimus from slavery.