A mysterious illness has been killing thousands of bats in New York and New England for the past two years, and it has now begun to spread to bats in New Jersey and Pennsylvania, environmental officials in those states said.

The disease causes bats to crawl out of their hibernation spots prematurely during the cold winter months and they end up dropping dead because they burn up crucial fat reserves. It also causes some bats to crawl further into cooler sections of their hibernation spots in an attempt to cool rising body temperatures, the New Jersey Star-Ledger reported.

Investigators have dubbed the phenomenon "white nose syndrome" because of a white fungus that develops around the many of the infected bats' noses and wing membranes. But what has been frustrating investigators is that the fungus is not found in all cases. It's not clear whether the fungus is the cause or a symptom of the ailment.

What is also worrying environmental officials is what effect the bats' deaths will have on the summer bug population, since bats eat insects.

Investigators think the disease is spreading so quickly because bats are highly migratory and intermingle with populations far away in other areas, and that when they hibernate, they cluster together in groups of up to 300, making an ideal habitat for the spread of pathogens.