We may be headed for a zombie apocalypse.

But unlike most zombie disaster plots, it’s undead viruses, not people, that are concerning experts.

Scientists have worried that the faster-than-usual melting of permafrost due to climate change would revive centuries-old viruses over the past few years. And now those worries are seemingly becoming a reality.

NPR reported that a 25-year-old teacher, Zac Peterson, on an archaeological dig in Alaska last summer contracted a skin infection, which he believes came from a pathogen that had been preserved in the permafrost he was excavating.

Take Action: Stand Up for the Arctic

And in 2016, scientists suggested that a sudden outbreak of anthrax in Siberia, Russia, had been caused by increased temperatures thawing permafrost and an anthrax-infected reindeer carcass from 1941, according to the Atlantic.

But not all experts are as convinced by these hypotheses.

Michael Zimmerman — a doctor who has studied mummified bodies for decades and has performed autopsies on bodies from the same area in which Peterson was excavating — told NPR he had actually tried to revive bacteria from an excavated frozen body to no avail.

"Nothing grew. Not a single cell,” he said. "We're dealing with organisms that have been frozen for hundreds of years. So I don't think they would come back to life."

But “zombie pathogens” are still not out of the question. 

In 2005, scientists were able to revive bacteria that had been frozen for 32,000 years and in 2007 brought bacteria that had been encased in ice for 8 million years back to life, the BBC reported.

Read more: Scientists Warn That the Arctic Might Not Be the Arctic for Very Long

Though the doctors did not test to confirm that Peterson had contracted “seal finger,” an infection that comes from handling seals’ body parts, he was successfully treated for the infection, NPR reported. Yet the only seals he came into contact with had been frozen for decades.

It’s clear that as climate change causes the earth to heat up, permafrost is melting at faster rates, revealing older layers of permafrost, the effects of which are yet to fully be seen.

Global Citizen campaigns for climate action, to preserve our planet and protect the environment. You can take action here to help combat climate change and global warming.