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Black History Month: Onesimus Spreads Wisdom That Saves Lives of Bostonians During a Smallpox Epidemic


René F. Najera, DrPH

February 3, 2019

In the early 1700s, about a century before , was going through New England and other American Colonies. In Massachusetts, colonists there saw smallpox arrive with cargo ships to Boston over and over again. There was not much the authorities could do beyond imposing quarantines and treating the sick.

This changed in 1721 thanks to the wisdom passed on from Onesimus, an African slave sold to Cotton Mather, an influential minister in Boston. (You might remember Mather from learning about the Salem Witch Trials.) Mather had bought Onesimus in 1706 and came to converse with him and learn about Onesimus' past. When Mather asked Onesimus if he had ever had smallpox back in Africa, Onesimus described the .

 consisted of first taking infectious material (like pus) from the blisters of smallpox patients. A healthy person then receives the material through a cut in the skin in a controlled manner and under the supervision of a physician. This was done so that the smallpox symptoms would be milder but still confer some sort of immunity in the future. Of course, the procedure was not without risk. People still developed severe symptoms and even died from smallpox via variolation, but those who died were in much smaller proportion to those who acquired it naturally from another person. (See how Benjamin Franklin reasoned it through and put his observations in a pamphlet: ).

After hearing Onesimus' story, Cotton Mather began to research the practice of variolation. He found that it was practiced in many parts of the world, not just Africa. Or, as he recorded in his diary, "the new Method used by the Africans and Asiaticks, to prevent and abate the Dangers of the Small-Pox, and infallibly to save the Lives of those that have it wisely managed upon them." Places like and had their own versions of variolation based on the same principle of exposing a person under controlled circumstances rather allowing them to contract it naturally. The practice was so effective in conferring immunity that African slaves sold in Massachusetts at the time were deemed to be more valuable if they bore the scar of variolation.

This research and correspondence with medical experts at the time encouraged Cotton Mather to push for variolation in the colonies. He burned some of his political and social capital in advocating for variolation before the next epidemic hit Boston. Needless to say, his proposal met with resistance. :

“As word spread of the new medicine, the people of Boston were terrified and angry. According to Mather, they “raised an horrid Clamour.” Their rage came from many sources; fear that inoculation might spread smallpox further; knowledge that the bubonic plague was on the rise in France; and a righteous fury that it was immoral to tamper with God’s judgment in this way. There was a racial tone to their response as well, as they rebelled against an idea that was not only foreign, but African (one critic, an eminent doctor, attacked Mather for his “Negroish” thinking). Some of Mather’s opponents compared inoculation to what we would now call terrorism—as if “a man should willfully throw a Bomb into a Town.” Indeed, one local terrorist did exactly that, throwing a bomb through Mather’s window, with a note that read, “COTTON MATHER, You Dog, Dam You; I’l inoculate you with this, with a Pox to you.”

In 1721, half of Boston's residents were infected with smallpox, about 11,000 people. Zabdiel Boylston, a physician who believed Cotton Mather and Onesimus on variolation, inoculated his own son and the slaves in his possession. The result was that one in forty people inoculated by Boylston died from smallpox. In those who acquired it naturally, one in seven died... A risk ratio of 5.7, meaning that people who acquired the disease naturally were almost six times more likely to die than those who acquired it by variolation.

At the end of the epidemic, 14 percent of the population of Boston was dead. Based on this experiment with variolation, the practice became more accepted in the colonies facing smallpox epidemics. By 1796, the vaccine based on cowpox would be developed by Edward Jenner. By the mid-1800s, variolation was discontinued in favor of immunization with cowpox as immunization was safer and more effective than variolation.

Onesimus would go on to partially purchase his freedom but still remaining in the service of Cotton Mather. His contribution to the understanding of smallpox and its prevention lives on today.


  • Cotton Mather Anguishes Over the Consequences of His Son's Inoculation Against Smallpox
  • T. E. C. Pediatrics May 1974, 53 (5) 756;
  • “How an African slave helped Boston fight smallpox” from The Boston Globe:
  • “How an African Slave in Boston Helped Save Generations from Smallpox” from


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