In 2008, scientists found three eastern massasauga rattlesnakes dead, their faces swollen and disfigured, in a forest near Carlyle, Illinois. It was the ninth year of a long-term monitoring program of the threatened species, and no one had ever seen anything like it before.

The faces of the reptiles were so messed up, Matthew Allender says, experts first thought they’d been run over by cars.

A close relative of the fungus O. ophiodiicola, which likely kills by overwhelming the snake’s immune system, had been found once before in a captive snake, but never in the wild. In the decade since the findings in Illinois, Allender and his colleagues have identified O. ophiodiicola in 25 snake