Scientists revived a 'zombie' virus frozen for 48,500 years in ice. They learned it could still infect other cells.

• Scientists revived a 48,500-year-old 'zombie' virus from permafrost and found it was still infectious.
• The virus was tested on amoebae but could indicate more dangerous viruses are lurking in permafrost.
• Some scientists are concerned that climate change thawing permafrost could reawaken ancient viruses.

From a horror movie plot to real life: Scientists have revived ancient "zombie" viruses from permafrost and discovered they could still infect living single-celled amoebae. The chances of these viruses infecting animals or humans are unclear, but the researchers say permafrost viruses should be considered a public health threat. 

Permafrost is a layer of soil that remains completely frozen year-round — at least it used to, before human activities started raising global temperatures. It covers 15% of land in the Northern Hemisphere.

Because of climate change, though, permafrost is thawing rapidly, unearthing a host of ancient relics from viruses and bacteria to wooly mammoths and an impeccably preserved cave bear.

According to CNN, French professor Jean-Michel Claverie found strains of the 48,000-year-old frozen virus from a few permafrost sites in Siberia. The oldest strain, which dated back 48,500 years, came from a sample of soil from an underground lake, while the youngest samples were 27,000 years old. One of the young samples was discovered in the carcass of a wooly mammoth.

Some scientists fear that as climate change warms the Arctic, thawing permafrost could release ancient viruses that haven't been in contact with living things for thousands of years. As such, plants, animals, and humans might have no immunity to them.

"You must remember our immune defense has been developed in close contact with microbiological surroundings," Birgitta Evengård, professor emerita at Umea University's Department of Clinical Microbiology in Sweden, told CNN.

"If there is a virus hidden in the permafrost that we have not been in contact with for thousands of years, it might be that our immune defense is not sufficient," she added. "It is correct to have respect for the situation and be proactive and not just reactive. And the way to fight fear is to have knowledge."

How 'zombie' viruses could infect hosts once they emerge

This isn't the first time Claverie has revived ancient viruses, or "zombie viruses" as he calls them. He's been publishing research on this topic since 2014 and says that beyond his work, very few researchers are taking these viruses seriously.

"This wrongly suggests that such occurrences are rare and that 'zombie viruses' are not a public health threat," Claverie and his colleagues report in their latest paper published February 18 in the journal Viruses.

In that study, Claverie and his team were able to revive several new strains of "zombie" viruses and found that each one could still infect cultured amoebas — a feat, Claverie said, that should be regarded as both a scientific curiosity and a concerning public health threat.

"We view these amoeba-infecting viruses as surrogates for all other possible viruses that might be in permafrost," he told CNN. "We see the traces of many, many, many other viruses. So we know they are there. We don't know for sure that they are still alive. But our reasoning is that if the amoeba viruses are still alive, there is no reason why the other viruses will not be still alive, and capable of infecting their own hosts."

The current research on frozen viruses like Claverie's 'zombie' virus is helping scientists understand more about how these ancient viruses function and whether, or not, they could potentially infect animals or humans.

Ancient bacteria like anthrax may already be thawing back to life

It's not just viruses. Ancient bacteria, too, could be released and reactivated for the first time in up to two million years as permafrost thaws.

That's what happened, scientists think, when outbreaks of the bacterial infection anthrax appeared in humans and reindeer in Siberia in 2016.

That may be a "more immediate public health concern," according to Calverie's paper.