He introduced his enslaver, Puritan clergyman Cotton Mather, to the principle and procedure of the variolation method of inoculation, which prevented smallpox and laid the foundation for the development of vaccines.
[1] Historian Ted Widmer of CUNY's Macaulay Honors College noted that "Onesimus reversed many of [the colonists'] traditional racial assumptions... [h]e had a lot more knowledge medically than most of the Europeans in Boston at that time.
[4] Mather followed Onesimus's medicinal advice because, as Margot Minardi writes, "inferiority had not yet been indelibly written onto the bodies of Africans.
"[14] Additionally, Mather believed that disease, specifically smallpox, was a spiritual and physical punishment, so he saw a cure as "God's providential gift", as well as a means of receiving recognition from New England society and re-establishing the influence of religious figures in politics.
[4] When Boston experienced a smallpox outbreak in 1721, Mather promoted inoculation as protection against it, citing Onesimus and African folk medicine as the source of the procedure.
[7] Doctors, ministers, laymen, and Boston city officials argued that the practice of inoculating healthy individuals would spread the disease and that it was immoral to interfere with the working of divine providence.
[15] The Acts and Resolves passed in Boston,[12] which included race-based punishments and codes to prevent enslaved or servant uprisings (because Bostonians feared conspiracy and conflict), showed a society skeptical of African medicine.
[15] In 1716, Onesimus attempted to buy his freedom from Mather, raising funds to "purchase" another enslaved man named Obadiah to take his place.
[15] Boston and London, in 1726 and 1722, respectively, performed trials on citizens, and, on average, inoculation decreased the mortality rate from 17% to 2% of the infected population.
[17] In 1980, the World Health Organization declared that smallpox had been completely eradicated due to global immunization efforts, making it the first and only human infectious disease for which this has been accomplished.