On Oct. 12, 2017, the New England Journal of Medicine published a small study in the letter-to-the-editors section. The media largely ignored it. They shouldn’t have. Because of this study, we might now have to change the name of what is arguably humankind’s most important medical advance.

Six researchers from around the world analyzed a vial of smallpox vaccine manufactured in 1902. They found that the origin of the smallpox vaccine might contradict history.

Easily spread by tiny droplets of saliva containing millions of virus particles, smallpox was a common, severe, debilitating disease. The virus, which causes a high fever and a permanently disfiguring, pus-filled rash, killed one in every three of its victims and blinded many survivors. According to Scourge: The Once and Future Threat of Smallpox by Jonathan Tucker, in 1492, when Christopher Columbus crossed the Atlantic Ocean, 72 million Native Americans lived in North America; by 1800, only 600,000 remained. Smallpox—brought by European settlers—killed most of the rest. Indeed, smallpox has killed more people than all other infectious diseases combined, Tucker wrote.

To understand why the New England Journal of Medicine study was surprising, we have to go back to the late 18th century to Edward Jenner, a country doctor working in southern England.

In 1768, when Jenner was 13 years old and training as an apprentice apothecary in Chipping Sodbury, England, he approached a young milkmaid who appeared ill. “Are you coming down with the smallpox?” he asked. “I cannot take that disease,” she said, “for I have had the cowpox”—a disease that caused blisters on cow udders. Sometimes milkmaids would get these same blisters on their hands. While training to be a physician in London years later, Jenner told fellow surgeon John Hunter about the milkmaid’s observation. Hunter encouraged him to test her theory.

On May 14, 1796, Edward Jenner got his chance. Sarah Nelmes, a milkmaid in his employ, developed cowpox blisters on her hands and wrists. Jenner removed the pus from one of the blisters and injected it into the arm of James Phipps, the 8-year-old son of a local laborer. Six weeks later, Jenner injected Phipps with dried crusts obtained from the healed blisters of a patient who had survived smallpox (called variolation) “in order to ascertain whether the boy, after feeling so slight an affection of the system from the cowpox virus, was secure from contagion of smallpox.” Variolation had been used for decades to prevent smallpox. Typically, variolation caused fever and a painful, ulcerating blister at the site of injection. But nothing happened to James Phipps. Jenner reasoned that Phipps’ lack of reaction to variolation meant that he would be protected against smallpox. Later, Jenner variolated Phipps 20 more times; each time Phipps survived without incident. Apparently, cowpox virus was similar enough to human smallpox so that inoculation with one protected against disease caused by the other.

Two years later, Jenner published his observations under the lengthy title “An Inquiry into the Causes and Effects of Variolae Vaccinae, a Disease Discovered in Some of the Western Counties of England, Particularly Gloucestershire, and Known by the Name of Cow Pox,” permanently affecting the way in which we referred to the procedure. The Latin word for cow is vacca; the possessive form of vacca is vaccinae. The phrase variolae vaccinae literally means “smallpox of the cow.”

And that’s how the word vaccine was born.

Within one year of Jenner’s publication, physicians had inoculated a thousand people with cowpox and translated Jenner’s observations into several languages. And 200 years later, smallpox—a disease that was estimated to have killed more than 500 million people, according to Tucker—was eradicated.

Which brings us back to the study from last month. The researchers who analyzed the vial of smallpox vaccine assumed that their phylogenetic analysis would reveal it to be primarily cowpox. What they found instead was that it was 99.7 percent horsepox, a virus that was also known to infect cows at the time that Edward Jenner was doing his experiments.

From a practical standpoint, this observation doesn’t have much impact. Because smallpox has been eliminated, we no longer use the smallpox vaccine. But one aspect of this vaccine remains—the word vaccine.

So, what now? Should we change the name of vaccines to something consistent with the finding that the original source might have been a horse? Should we call the procedure “equination”? And should we call those who oppose vaccines “anti-equinators”? Probably not. But this recent finding was certainly a surprise.