Meredith’s discovery generated enough publicity that members of the public started sending her pictures of squirrels from their own backyards, some of whom had the same lesions. Most of the shots came from Scotland, but one was from Brownsea Island—an island off the southern coast of England, and some 480 miles away from Edinburgh. “Someone had done a day trip there, seen a squirrel, and said: Is this leprosy?” says Meredith. “I looked at it and said: Wow, it’s identical!”

Working with Stewart Cole, from the Swiss Federal Institute of Technology in Lausanne, Meredith analyzed the cadavers of 110 red squirrels from Great Britain and Ireland, and found that almost a third of them had leprosy, including several without any clinical signs.

Those in Scotland, Ireland, and the Isle of Wight were infected with Mycobacterium lepromatosis—a second species of leprosy bacterium that was only identified in humans in 2008. By contrast, all the squirrels from Brownsea Island had M. leprae, the more traditional leprosy microbe.

By comparing the microbes’ genomes, the team showed that the M. leprae strains currently infecting the Brownsea squirrels are almost identical to those recovered from a medieval human skeleton buried in the nearby city of Winchester some 730 years ago. So it’s possible that humans passed M. leprae to red squirrels several centuries ago, and that the Brownsea individuals have harbored the microbe long after its eradication on the mainland.

Could the squirrels ever pass the disease back to us? Armadillos certainly do in the United States, but for the moment, there’s no evidence of squirrel-to-human transmission in Britain. “It’s not impossible, but there’s no evidence that we’re at risk,” says Meredith. “We’re more concerned about the squirrels.”

Red squirrels are an endangered species in Britain. Once common, they’ve had to contend with the introduction of gray squirrels from the Americas, which outcompeted them and infected them with squirrelpox—an often fatal disease. As a result, the country is currently home to more than 2.5 million grays but just 140,000 reds, most of which live in Scotland.

Those that get leprosy can live for many years with the condition, but Meredith wants to see if their health suffers in the long run. They might eventually die because they’re unable to feed properly, or because leprosy makes them more vulnerable to other infections. (Contrary to stigma, leprosy doesn’t make body parts fall off; instead, deadened peripheral nerves sometimes stop people from noticing injuries or infections in their extremities, leading to eventual amputations.)

Why did the squirrels become infected in the first place? They belong to a different order of mammals than either humans or nine-banded armadillos, and all three species are separated by around 100 million years of evolution. And yet the three of us, out of all the mammals in the world, are the only ones know to harbor M. leprae. And for that matter, why does the red squirrel get infected when the closely related grey squirrel doesn’t seem to?