The news was terrifying to the colonialists of Massachusetts. Smallpox had made it to Boston and was spreading very rapidly. The first victims, who were passengers on a ship from the Caribbean, were locked up in a house identified only by a red flag that reads “God have mercy on this house.” Meanwhile, hundreds of residents of the busy colonial town had started to run for their lives. Many were terrified of what might happen if they were exposed to the deadly virus.

They had every reason to fear. The virus was extremely contagious and was spreading like wildfire in large epidemics. Smallpox patients experienced fever, fatigue, and a crusty rash that could leave disfiguring scars on the body. In up to 30 percent of cases, the virus killed its victims.

But the smallpox epidemic of 1721 was different from any that came before it. The sickness swept through the city, killing hundreds. This was a period before modern medical treatment or a robust understanding of the infectious disease. But an enslaved man known only as Onesimus suggested a potential way to keep people from getting sick. Fascinated by Onesimus’ idea, a brave doctor and an outspoken minister carried out a bold experiment to try to stop the deadly smallpox.

Smallpox was one of that era’s deadliest diseases. Historian Susan Pryor notes that “Few diseases at this time were as universal or fatal.”

The European colonialists saw effects of smallpox not just among their own countrymen, but among the Native Americans to whom they introduced the disease. Smallpox destroyed Native American communities. And with no immunity, they were unable to fight off the virus. Today, the death toll of the native of Americans due to smallpox has been fingered as a deliberate attempt by colonialists to decimate them.

The Smallpox virus also entered the town through slave ships. Sometimes it was transmitted by African slaves who were packed in unsanitary quarters. The infected among them passed the disease along to one another and, eventually, to colonialists and inhabitants at their destinations. One of those destinations was Massachusetts, which was then a center of the early slave trade, in America. The first slaves were known to arrive in Massachusetts in 1638, and by the year 1700, about 1,000 slaves lived in the colony, most in Boston.

In 1706, an enslaved West African man was purchased for Cotton Mather, who was a prominent Puritan minister. Cotton Mather gave the slave the name Onesimus, after a Biblical slave whose name meant “useful.” Mather, who was a powerful figure in the Salem Witch Trials, believed that slave owners had a religious duty to convert slaves to Christianity and also educate them. But like other white men of his generation, he looked down on what he called the “Devilish rites” of Africans and was worried that the slaves might openly rebel.

Cotton Mather didn’t trust his slave Onesimus. He wrote about having to monitor him carefully due to what he thought was his “thievish” behavior. He also recorded in his diary that he was a “wicked” and “useless” slave. But in 1716, all that bad blood towards Onesimus would change when he told Cotton something he did believe: That he knew how to prevent the smallpox disease.

Onesimus, who “is a pretty intelligent fellow,” Cotton Mather later wrote, told him he had had smallpox—and then later didn’t. Onesimus told Cotton that he “had undergone an operation, which had given him something of the smallpox and would forever preserve him from it...and whoever had the courage to use it was forever free of the fear of contagion.”

The operation Onesimus referred to involved rubbing the pus from an infected person into an open wound on the arm of a non-infected person. Once the infected pus was introduced into the body, the person who underwent the procedure was automatically inoculated against smallpox. This procedure wasn’t necessarily a vaccination, which involves exposure to a less dangerous virus to provoke immunity. But it did trigger the recipient’s immune response and protected the people against the disease most of the time.

Cotton Mather was fascinated at the efficacy of Onesimus’s remedy. He soon verified Onesimus’ story with that of other African slaves and also learned that the practice had been used in Turkey and China. He then became an evangelist for inoculation which was also known as variolation, and then spread the good news throughout Massachusetts and other places, hoping it would help prevent the deadly smallpox.

But Cotton Mather hadn’t bargained on how unpopular the treatment procedure would be. The same discrimination that caused him to distrust his servant made other white colonists reluctant to undergo a medical procedure developed by or for black people. Cotton Mather “was vilified,” historian Ted Widmer once told WGBH.  He said that “A local newspaper, called The New England Courant, ridiculed him. An explosive device was thrown through his windows with an angry note. There was an ugly racial element to the anger.”

Religion also contributed to the refusal of the treatment by the colonialists. Other preachers argued that it was against the will of God to expose his creatures to such dangerous diseases.

But in 1721, Mather and Zabdiel Boylston got their chance to test the power of inoculation. Bolyston was the only physician in Boston who supported the technique.

That year, a smallpox epidemic spread from a ship to the population of Boston, infecting about half of the city’s residents. Boylston sprang into action, inoculating his son and his slaves against the smallpox disease. Then, he began inoculating other willing people in Boston. Of the 242 people he inoculated, only six died—one in 40. But among those who refused to take the treatment, one in seven died.

The smallpox epidemic killed 844 people in Boston. This was over 14 percent of the population of the city. But the treatment and the results of that epidemic had yielded hope for future epidemics. It also helped set the stage for vaccination against viruses. In 1796, Edward Jenner created an effective vaccine that used cowpox to provoke smallpox immunity. Interestingly, it worked. Eventually, smallpox vaccination then became mandatory in Massachusetts.

Did Onesimus live to see the success of the technique he introduced to Cotton Mather? Well, that isn’t clear to history. Nothing is really known of his later life other than that he partially purchased his freedom from Cotton Mather. To do so, he gave Mather money to purchase another slave.

What is clear to historians is that the knowledge he passed on saved hundreds of lives—and led to the eventual eradication of smallpox.

In 1980, the World Health Organization declared smallpox entirely eradicated from the world due to the spread of immunization worldwide. It remains the only infectious disease to have been entirely wiped out of the surface of the earth.

Onesimus’s instincts and knowledge of medicine points to the historical facts that Africans invented medicine and are that they have been exposed to science through their indigenous cultures and religions.

Americans and indeed the world needs to recognize the medical contributions of men such as Onesimus, although it is no surprise that his name has been swept under the rug, with the system giving credit to those that learned from him.