In 2006, Anna Meredith came across a dead red squirrel with a weird skin disorder. Its ears lacked the characteristic red tufts, and were instead swollen, smooth, and shiny, like the cauliflower ears of boxers and rugby players. Its nose, muzzle, and eyelids were similarly swollen and hairless. Meredith, a professor of conservation medicine at the University of Edinburgh, had never seen anything like this before.

But she soon saw the same problems again—in six more squirrels over the next six years. She and her colleagues analyzed tissue samples from the dead animals. And to their surprise, they discovered that the squirrels had leprosy.

That’s astonishing for two reasons. First, even though leprosy still affects at least 385,000 people around the world (including a few hundred in the U.S.), the disease was eradicated from Britain several centuries ago. Second, squirrels aren’t meant to get leprosy.

The disease is mainly caused by a bacterium called Mycobacterium leprae, which attacks the skin and peripheral nerves. Chimps and some monkeys can occasionally catch it from people, but until now, scientists knew of only two species that naturally harbor the disease: humans, and nine-banded armadillos in the southern United States. The latter actually acquired the disease from the former; European settlers brought leprosy to the New World and then passed it onto armadillos several centuries ago.

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?

No one knows. Meredith’s team looked at an immune gene called TLR1. A few mutations in this gene have been linked to either a greater or lower risk of leprosy in humans, but none of these specific mutations are found in either armadillos or squirrels. Some squirrels did seem to have their own TLR1 mutations that reduced their risk of infection, but with such a small sample, it’s hard to say for sure.

Perhaps the weird troika of host species simply reflects our ignorance about where leprosy hides. It may be lurking in more animals than we realize. “We need to look more closely at the possibility of a wildlife reservoir,” says Meredith.

And “there is circumstantial evidence that M. leprae has an environmental reservoir,” says Helen Donoghue, from University College London. That is, the microbe might hide out in water, soil, vegetation, or something else. Perhaps that’s how the red squirrels originally became infected, she adds. The idea of an environmental reservoir has been long disputed, but perhaps needs to be revisited in light of the squirrel discovery.

“Although leprosy has been known since biblical times, and still remains a major public health problem in many parts of the world, our understanding of how this infection transmits and causes disease is very limited,” says Anura Rambukkana from the University of Edinburgh. And since the infected squirrels develop symptoms that “somewhat resemble human leprosy”, he adds, they may help us to understand how the disease manifests in humans, and how it spreads between us.