Enslaved Human Being • Medical Knowledge Bearer • Inoculation Advocate 

The two images connect an African medical tradition carried to colonial America with the growing acceptance of smallpox inoculation in the years that followed. The portrait represents Onesimus and the knowledge he shared about a practice he had learned in Africa, while the historical letter illustrates how inoculation later became important enough to be incorporated into efforts to protect soldiers from smallpox. Together, the images represent more than the history of a medical procedure—they reveal how knowledge carried by an enslaved African entered colonial medical practice and contributed to a broader understanding of disease prevention. His story begins with knowledge that had traveled across the Atlantic long before it was recognized by American medicine.

Overview:

But for this man!

Onesimus was an enslaved African (ca. 1680s - after 1721) man whose medical knowledge helped change the course of American history. He was likely born among the Akan people in what is now Ghana and was brought to Boston aboard a slave ship in the early 1700s. In 1706, Puritan minister Cotton Mather enslaved him. Around 1716, Onesimus explained to Mather a medical practice long practiced among sub-Saharan Africans, later called variolation by the colonists. The process involved placing material from an infected smallpox sore into a cut on the skin of a healthy person to create immunity against the deadly disease.  Essentially, Onesimus was introducing the science of inoculation to the British colonies in America.

During Boston’s 1721 smallpox epidemic, Dr. Zabdiel Boylston used the method on patients despite public anger and fear. The success of inoculation helped save countless lives in Boston, and later influenced the spread of inoculation in the American colonies.  

During the early years of the war, a catastrophic 90% of casualties in the Continental Army were from infectious diseases, with the lion’s share of those from smallpox.  During that period smallpox had a mortality rate of about 30%. George Washington was initially against inoculation, until he realized that without it the British might win the war.  Without Onesimus, the US might not have ever become an independent nation.

Quick Facts:

• Born ca. 1680s

• Likely born among the Akan people of present-day Ghana

• Brought to Boston on a slave ship in the early 1700s

• Enslaved by Puritan minister Cotton Mather in 1706

• Shared African knowledge of smallpox inoculation with Mather

• The procedure later became known as variolation

• Helped influence Boston’s response to the 1721 smallpox epidemic

• Dr. Zabdiel Boylston used the method on hundreds of patients

• Inoculation later influenced George Washington’s military policy

• Onesimus was able to purchase partial freedom from Cotton Mather

• His birth name, birth date, and death date remain unknown

• Death after 1721

Why This Story Matters:

Onesimus matters because his knowledge helped introduce one of the earliest forms of immunization to colonial America. At a time when many Europeans distrusted African knowledge and viewed enslaved people as inferior, Onesimus shared a medical practice that proved more effective than any colonial treatments. His contribution helped reduce deaths during smallpox outbreaks and changed how Americans thought about disease prevention. Yet for generations, many history books focused more on Cotton Mather and other white leaders than on the enslaved African man who first explained the process. Onesimus’s story challenges the idea that only wealthy or powerful white men shaped early American history. His life reminds us that enslaved Africans contributed knowledge, science, and innovation that helped build and protect the nation long before they were granted freedom or citizenship.  

Though Onesimus left behind no written books or speeches, his explanation of African inoculation practices helped launch one of the earliest public health campaigns in colonial America. Dr. Zabdiel Boylston used the procedure on hundreds of patients during the outbreak. While many Bostonians opposed the practice and even threatened violence against supporters, the results showed that inoculated patients died at much lower rates than those who caught smallpox naturally. The success of variolation helped change medicine in the colonies and laid the groundwork for modern vaccines. 

Legacy:

The legacy of Onesimus reaches far beyond colonial Boston. His knowledge helped normalize inoculation in America decades before modern vaccines were invented. Historians believe the growing acceptance of inoculation later influenced George Washington’s decision to require smallpox inoculation for Continental soldiers during the Revolutionary War. Some scholars argue that this decision helped the colonies survive the war and secure independence. Onesimus’s story also exposes how history often overlooks Black contributions, especially when those contributions came from enslaved people. Today, many educators and historians see Onesimus as an important figure in the history of medicine, public health, African knowledge systems, and the hidden contributions of Black people in the founding of the United States. 

Discussion Questions:

1. Why do you think many Americans know the names of George Washington and Cotton Mather, but not Onesimus, even though his knowledge helped save lives?

2. How might American history look different if schools spent more time teaching about the scientific and medical knowledge brought by Africans to the colonies?

3. Onesimus was enslaved and denied freedom, yet his ideas helped protect thousands of people. What does this reveal about who is allowed to be seen as an “American hero”?

Reflection Prompt:

1. Why might some stories about the founding of America focus mainly on soldiers, presidents, and politicians while leaving out people like Onesimus, whose contribution came through science, medicine, and African knowledge? 

Sources & Further Reading:

Sources:

1. “Onesimus.” BlackPast.org. 

2. “Race and Medicine in Nineteenth- and Early-Twentieth-Century America.” U.S. National Library of Medicine, National Institutes of Health. 

3. “About Edward Jenner.” Jenner Institute, University of Oxford. 

4. “Smallpox, Inoculation, and the Revolutionary War.” National Park Service, U.S. Department of the Interior. 

5. “Variolation vs. Vaccination: 18th Century Developments in Smallpox Inoculation.” Massachusetts Historical Society. 

6. “Smallpox Inoculation Letter.” Document. George Washington to Lt. Colonel David Grier, March 12, 1777. George Washington’s Mount Vernon. 

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