One cave had 15,584 bats in 2005, 6,735 in 2007 and an estimated 1,500 this winter. Another went from 1,329 bats in 2006 to 38 this winter. Some biologists fear that 250,000 bats could die this year.

Since September, when hibernation began, dead or dying bats have been found at 15 sites in New York. Most of them had been visited by people who had been at the original four sites last winter, leading researchers to suspect that humans could transmit the problem.

Details on the problem in neighboring states are sketchier. “In the Berkshires in Massachusetts, we are getting reports of dying/dead bats in areas where we do not have known bat hibernacula, so we may have more sites than we will ever be able to identify,” said Susi von Oettingen, an endangered species biologist with the United States Fish and Wildlife Service.

In Vermont, Scott Darling, a wildlife biologist with the Fish and Wildlife Department, said: “The last tally that I have is approximately 20 sites in New York, 4 in Vermont and 2 in Massachusetts. We only have estimates of the numbers of bats in the affected sites  more or less 500,000. It is impossible for us to count the dead bats, as many have flown away from the caves and died  we have over 90 reports from citizens across Vermont  as well as many are still dying.”

People are not believed to be susceptible to the affliction. But New Jersey, New York and Vermont have advised everyone to stay out of all caverns that might have bats. Visitors to affected caves and mines are asked to decontaminate all clothing, boots, ropes and other gear, as well as the car trunks that transport them.

One affected mine is the winter home to a third of the Indiana bats between Virginia and Maine. These pink-nosed bats, two inches long and weighing a quarter-ounce, are particularly social and cluster together as tightly as 300 a square foot.

Image DIRE EFFECTS Ryan von Lindin, a biologist (with Tina Kelley of The New York Times), in a mine in the Adirondacks. Credit... Al Hicks

“It’s ironic, until last year most of my time was spent trying to delist it,” or take it off the endangered species list, Mr. Hicks said, after the state’s Indiana bat population grew, to 52,000 from 1,500 in the 1960s.