In 1796, English physician Edward Jenner had a hunch. As hundreds of thousands around him succumbed to the blistering agony of smallpox, Jenner’s eye was on the rosy, unblemished complexion of Sarah Nelmes, a local milkmaid. Her skin bore none of the oozing lesions that signaled almost certain death for a third of smallpox sufferers—save for her hands, busily milking a cow named Blossom.
The locals called the unsightly affliction “cowpox.” But apart from a smattering of welts, Sarah and her fellow milkmaids were remarkably free of disease. To Jenner, this wasn’t a coincidence. Acting on little more than sparse observations, Jenner decided to extract a small sample of Sarah’s pus and inject it into the arm of a young boy named James Phipps. To everyone’s amazement—including Jenner’s—when Jenner stuck Phipps with a second needle, this time sporting a hefty dose of smallpox, Phipps remained healthy. Against all odds, the risky treatment had granted the child miraculous immunity.
This revolutionary (and wildly unethical) experiment catapulted the world into the era of what would come to be called “vaccination,” a word whose etymology contains a reverent nod to the Latin vacca, for “cow.” Smallpox would become the first disease officially conquered by human medicine.
“[Smallpox eradication] shows what we can really achieve when we have the proper tools to fight a disease,” says Sabrina Sholts, a curator of anthropology at the National Museum of Natural History, where she developed the infectious-disease-centric “Outbreak” exhibit. “But knowing about the disease also shows us how many people were killed… and we realize there’s no reason it couldn’t happen again.”
Not only is there the potential for smallpox (or at the very least, something very similar) to resurge, but unbeknownst to most, the very origins of this revolutionary vaccine are in question. For decades, scientists have thought the legend of cowpox as the savior—first, of James Phipps, then, of the world—may very well be wrong. That cryptic crack in medical history could leave humanity vulnerable to a future outbreak—however unlikely it may be.
By the end of the 18th century, smallpox, caused by variola virus, was a worldwide terror, claiming hundreds of thousands of lives each year. The disease was an ancient one, plaguing populations at least as far back as the third century B.C., and infected indiscriminately, sparing neither rich nor poor and felling men, women and children alike.
So, when Jenner’s “vaccine” (really just pus teeming with virus) hit the scene, it literally went viral. By 1813, it was widely accessible in Europe, Asia and the Americas. Because poxviruses like smallpox are most easily passed from animal to animal, scientists first created new reserves of the vaccine by collecting pus from infected livestock. Each time stocks ran low, scientists rounded up a fresh herd of naturally infected animals and consolidated their secretions.
“The vaccine evolved,” says Inger Damon, who leads the poxvirus and rabies branch at the United States Centers for Disease Control and Prevention. “The way it was propagated—either on the sides of cows or in rabbits—changed and introduced related viruses into the pool… [so] a swarm of viruses was used in various vaccines.”