(credit: hehaden)

Those Brits, they just love their cute red squirrels. They think it’s a bloody shame that the more aggressive gray squirrels have pretty much taken over. The grays were brought to England from the US as playthings for 19^th century noblemen because they encapsulated the American pioneer spirit. But now it looks like these imports might have been an inadvertent act of disease prevention.

Red squirrels are now protected in the UK as the grays have completely infiltrated their habitat. But there’s also a disease element to this struggle, a poignant rodent reversal of Europeans’ inadvertently killing off Native Americans with the smallpox they carried. Gray squirrels are immune to the squirrel pox virus that kills the native red squirrels, but they can still carry it.

Over the centuries, of course, England has faced more dire threats than invasive gray squirrels. One of these was leprosy. It had generally been thought that leprosy could be transmitted only between humans—that it had no other host to hide in. And it was also thought to be caused by only one infectious agent: Mycobacterium leprae.