In the winter of 2006, biologists in New York State got a gruesome surprise. As they surveyed colonies of hibernating bats, they discovered heaps of dead animals on the floors of caves and abandoned mines.

The culprit was a fungus new to science. It caused white-nose disease, named for the fuzzy pale tendrils that sprouted from the nostrils of its victims. (The disease was originally known as white-nose syndrome, but was renamed in recent years.) The fungus, Pseudogymnoascus destructans, or P. destructans, has spread from New York to 40 states and nine Canadian provinces.

“This is the most dramatic wildlife mortality event that’s ever been documented from a pathogen,” said DeeAnn Reeder, a disease ecologist at Bucknell University. “Millions and millions and millions of animals have died.”

In recent years, bat experts have gained some guarded optimism. They have found ways to protect bats from white-nose disease and to help infected animals survive. But a new study published on Wednesday raised the possibility that North American bats could get slammed by a second wave of white-nose disease.

An extensive genetic survey has found that Pseudogymnoascus destructans is actually two species native to Europe and Asia. Only one has reached North America. If the second one is introduced to the continent, it could start another devastating epidemic.

“It’s like a reboot,” said Dr. Reeder, who was not involved in the study. “I think it’s terrifying, honestly.”