In December 1706 the congregation of Boston’s North Church bought a young West African man, likely of the Akan or Coromantee people, and gave him to their minister, Cotton Mather, who named him Onesimus, meaning useful. Mather taught him to read and write. Around 1716, asked whether he had ever had smallpox, Onesimus answered yes and no, described variolation as practiced across Sub-Saharan Africa, and showed the scar on his arm. Pus from a mild case was rubbed into a cut on a healthy person to produce a small infection and lifelong immunity. Mather wrote it up for the Royal Society of London.

When smallpox reached Boston on a ship from the Caribbean in 1721, Mather urged mass inoculation and the city turned on him. Doctors called it murder, ministers called it defying God, and pamphlets called it an African plot. Someone threw a bomb through his window. Dr. Zabdiel Boylston inoculated his six year old son and two enslaved people, then 280 people in all, with about 2.2 percent dying against 14.3 percent among the roughly 5,889 natural cases. The episode also traces Onesimus’s private losses and the price Mather set on his freedom.

• Onesimus’s son died in 1714 and his daughter Katie died of tuberculosis
• He refused to convert to Mather’s faith, and Mather complained of his stubborn behavior in his diary
• To buy his freedom he had to purchase another enslaved man, Obadiah, as his replacement and pay Mather five pounds more
• Edward Jenner built on variolation with his cowpox vaccine in 1796, and smallpox was declared eradicated in 1980
• Boylston’s tombstone credits him with introducing inoculation, while Onesimus was named one of the 100 best Bostonians in 2016