White-nose syndrome (WNS), the disease rampaging its way through the bats of North America, is caused by a fungus, scientists have confirmed.
Researchers from a number of US institutions infected healthy bats with the fungus Geomyces destructans, and found they did develop the disease.
The team also showed that the fungus can pass from one bat to another.
Writing in the journal Nature, they say WNS "has the potential to decimate North American bat populations".
So far, the disease has killed more than a million bats in the eastern US and Canada since it was first identified in New York state in 2006.
G. destructans is usually found on the animals' snouts, where it causes a characteristic white colouration and, more significantly, lesions in the skin.
Although the fungus has been suspected as the disease's primary cause, researchers have not been able to prove it definitively.
Animals sometimes contract fungal diseases when their immune systems have been compromised, perhaps by a different infection, and this has been seen as a possibility for WNS as well.
The research group, led by David Blehert from the National Wildlife Health Center at the US Geological Survey, appears to have proven the primary role of the fungus.
No 'magic bullet'
First, they took little brown bats (Myotis lucifugus) that were fit and healthy, and infected them with G. destructans from culture.
Three months after infection, all showed lesions characteristic of WNS.
To determine how the fungus could be transferred from one bat to another, the researchers set up two different experiments.
In one, infected bats could mingle with healthy ones. Nearly 90% of the healthy ones had contracted fungal infection three months on.
In the wild, bats appear to transmit the fungus when they "swarm" in vast groups outside the caves where they will hibernate, literally rubbing shoulders and everything else with their fellows, who may be from a different species or a different cave.
In the other experiment, healthy bats and diseased ones were put in neighbouring cages separated by 1.3cm.
Here, the fungus did not spread, indicating that infectious spores are not airborne.
The researchers hope that confirming the disease's cause will enable agencies to concentrate on ways to halt its advance.
"There's unlikely to be any 'magic bullet' that we could deploy to block the disease - diseases among free-ranging wildlife are not usually stopped in their tracks when they're established," Dr Blehert told BBC News.
But measures could be adopted to reduce transmission by humans, he said, by closing caves and insisting on decontamination when people do go into hibernation sites.
This would probably not stop the disease spreading between neighbouring bat populations that will normally intermingle.
But it could stop people inadvertently taking spores from one side of the continent to another, or even to other continents.
It is generally believed that WNS came to the US this way from Europe, where bats appear to be immune.
Another approach might be to change the environment of the cave subtly, so as to slow the fungal spread while leaving it habitable for bats and other wildlife.
G. destructans appears to like low temperatures and certain levels of humidity; so altering those parameters could retard its growth.
Meanwhile, research goes on into how the fungus actually kills the animals.
"It might be called white-nose syndrome, but when you look closely the most significant damage perhaps is to the wings," said Dr Blehert.
"These are exquisite organs, external structures made entirely of skin, comprising eight times more skin than is on the rest of the body.
"They play a role in water balance when the bats are hibernating, blood pressure regulation, even passive CO2 exchange with the atmosphere - so if the wings are heavily damaged by the fungus, that might explain its lethality."
The advance of WNS across North America in some ways parallels the spread of chytridiomycosis, the frequently lethal fungal disease affecting amphibians.
As with chytrid, some species of bat appear immune, or at least resistant. In North America, these include the gray bat and the spectacular Virginia big-eared bat.
Scientists want to understand why they are immune when others such as the little brown bat succumb; and, as is being investigated with chytrid, to see whether this natural immunity can be turned into a defence for vulnerable species.
Chytridiomycosis has wiped many amphibian populations and quite a few entire species off the map; and this research group warns of similar troubles ahead for bats unless WNS is tackled.
"Fungal pathogens have the unique capacity to drive host populations to extinction because of their ability to survive in host-free environments," they write.
"Given the high mortality rate and speed at which WNS has spread, the disease has the potential to decimate North American bat populations and cause species extinctions similar to those documented for amphibians affected by chytridiomycosis."
Follow Richard on Twitter