White-nose syndrome, the deadly bat disease that has killed more than 6 million cave-dwelling bats in the US, continues to spread across North America, with 25 states and five Canadian provinces reporting the disease in bat caves since it was first documented in 2006.

In February of this year, white-nose syndrome (WNS) was reported in Kentucky's Mammoth Cave National Park, and park officials said they observed atypical bat activity during the winter months.

"We have observed some increase in bat activity, which may be due to the illness," said park superintendent Sarah Craighead. "We have also found several dead bats in the last few weeks."

The disease, characterized by fuzzy white mold on bats' faces, was located in remote sections of Mammoth Cave.

A report by The Associated Press this week revealed that visitors touring the park, which is the largest known cave system in the world, are now required to walk across bio-security mats upon exiting the cave system in an effort to prevent the spread of the disease. WNS is not believed to pose a risk to humans, although park officials warn visitors not to touch bats and to advise an official if contact is made.

WNS is a disease that affects hibernating bats, and once a colony is infected by WNS death is almost sure to follow. Researchers have pinpointed a fungus called Pseudogymnoascus destructans as the source of the disease.

Bats infected with WNS exhibit uncharacteristic behavior during the winter months when they would naturally be hibernating, including flying outside during the day. This activity during a time typically spent hibernating drains the bats of precious energy reserves, leading to their death in the winter.

Some hibernacula infected with WNS experience 90-100 percent mortality rates, according to White-NoseSyndrome.org, a national WNS information repository.

"This is by far the largest threat to that we've had for bats, probably in the history of this country," Mammoth Cave scientist Rick Toomey told the AP.

WNS continues to spread rapidly throughout parts of the US; it has recently reached a cave near Atlanta, marking the farthest point south WNS has been found, the AP reported.

Earlier this month officials in Michigan and Wisconsin reported WNS within state boundaries for the first time.