Ed Yong in The Atlantic:
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.