They are fungi, and they arrived in the United States from overseas with an assist from humans — through travel and trade. They prefer cold conditions and kill with precision, so efficiently that they’re creating a crisis in the wild.

The death toll on amphibians, bats and snakes from fungi represents “potential extinction events,” said [Dr. Jeremy] Coleman, a U.S. Fish and Wildlife research biologist who coordinates the government’s response to the bat-killing infection known as white-nose syndrome. It’s so large, he said, that it can’t be measured “as far as numbers of dead organisms,” and is “decimating populations as we know them.”