The next flu pandemic may be hibernating in an Arctic glacier or frozen Siberian lake, waiting for rising temperatures to set it free. Then birds can deliver it back to civilization.

New research suggests an influenza virus could go into hiding in the ice when earlier generations of humans, birds or other hosts developed immunity strong enough to drive the virus to extinction. It’s a sort of evolutionary loophole.

“It can bring a set of viral genes back to life that have been frozen for centuries or thousands of years,” said environmental biologist Scott Rogers of Bowling Green State University in Ohio. “If hosts haven’t seen the virus in a while, then there may be no active immunity.”