Answer
The variolation method, a precursor to vaccination, was introduced to Cotton Mather by Onesimus, an enslaved African man in his household. Mather observed Onesimus' immunity to smallpox after being inoculated in Africa and recognized the potential of this technique to combat the deadly disease. Through his writings and advocacy, Mather played a crucial role in popularizing variolation in the American colonies, contributing to the fight against smallpox.