In November 2019, 50 scientists from around the world assembled at Herrenhausen Palace in Hannover, Germany, to talk through an emerging threat to public health: "zombie" viruses and microbes emerging from the thawing ground.

The frozen earth that covers much of the Arctic is home to growing microbial communities. For centuries, they had lain dormant, barely active or completely suspended, subsisting on minuscule pockets of water squeezed between the ice. With the Arctic warming at two to five times the global average, those pockets are becoming pools; rivulets, rivers; and puddles, ponds. The Arctic is waking up, and the microscopic organisms embedded in the land are coming back to life.

The scientists in Germany agreed the climate is warming and the permafrost is thawing. But they wanted to know what it all means for humans and the future of infectious disease.

"The impetus from the meeting was determining what’s going to thaw out of the permafrost and kill us," says Susan Kutz, a professor of ecosystem public health at the University of Calgary and one of the scientists at the Hannover meeting.

‘A gigantic reservoir of ancient microbes or viruses’

In a 2017 paper, a team of Belgian researchers describe the threats to human health from microbes that were previously frozen in permafrost.

"Over the past few years, there has been increasing evidence that the permafrost is a gigantic reservoir of ancient microbes or viruses that may come back to life if environmental conditions change and set them free again," the authors write.

The paper describes a separate study in which two viruses emerged from a single sample of 700-year-old caribou droppings. They were both able to be resurrected.

In 2014, scientists discovered a giant virus (a classification only discovered a decade earlier) frozen in a 30,000-year-old ice core. Like a scene out of a sci-fi movie, the scientists thawed it and watched it take over an amoeba.

The scientists concluded in a paper that their ability to resurrect the virus suggests that thawing permafrost — as a result of global warming or industrial exploitation of circumpolar regions — might pose a threat to human or animal health.

Evolutionary ecologist Ellen Decaestecker, who co-authored the 2017 paper, says the increasing encroachment of people into natural areas worldwide is presenting new opportunities for health crises.

"We are changing the environment very fast at this moment in terms of habitat fragmentation and climate change," she says, adding that people are also travelling more and more (or at least they were before COVID-19 hit). "The chance that [an outbreak] happens as a result of the combination of these factors is quite high."

The viruses and microbes may also present another problem: they could contain the blueprints for resistance to antibiotics or other medicines. If given the chance, they could share that information with their modern relatives.

The world became aware of the infectious risk in the permafrost when an outbreak of anthrax occurred in the Yamal Peninsula in Siberia during the warm summer of 2016. Thousands of reindeer and a child died, while dozens of people were hospitalized with bacterial infections. Headlines blared that this was the start of a new wave of frozen diseases that would not only reawaken but infect and kill people.

The reality, as is often the case, is a little more nuanced.

A recently published paper suggests that a Russian anthrax vaccination program for reindeer that was halted in 2007 probably played a bigger role in the outbreak than a warm summer. The reindeer that died may have been the first cohort without the vaccination to be exposed to the bacterium, which can survive for hundreds of years in the soil.

While the permafrost theory shouldn’t be discarded entirely, it "might be a bit oversimplified," says study lead author Karsten Hueffer, a veterinary microbiologist at the University of Alaska Fairbanks.

But the risk of thawing permafrost runs deeper than the re-emergence of old diseases: a warmer Arctic brings brand new problems with it.

‘Bigger fish to fry’

Zombie viruses make for attention-grabbing headlines. But for the people living in the Arctic, infectious diseases that come from more mundane sources could pose a much greater threat.

"I really think that with climate change, we probably have — to put it flippantly — bigger fish to fry here," Hueffer says.

Climate change and human intrusions are changing the landscape, opening up new ways for microbes to get around and infect animals and humans.

New roads, new mines and new drilling programs are bringing more people to the Arctic than ever before, just as the soil is beginning to offer a multitude of freshly virulent germs.

The same warming is inviting new species north — some of which are hosts for pathogens that can infect humans. Drinking water straight out of Arctic streams and lakes, common practice in many places, is becoming more risky as beavers push farther into the North. Beavers are hosts to parasites like Giardia, which causes "beaver fever," a painful, diarrhea-inducing abdominal sickness. Mosquitoes carrying West Nile virus are being found farther and farther north as well. This is adding stress to the thinly spread medical systems of the North.

"I’m concerned about the fact that we don’t understand — and we very, very likely underestimate — the effect of infectious disease on wildlife," Kutz says.

If wildlife is affected, humans can be affected, too. Diseases can jump from animals to humans and deplete animal food sources people rely on.

Almost every herd of caribou, for instance, is declining precipitously across North America. Kutz says the role infectious disease is playing in that decline may have been overlooked, and climate change is feeding the fire.

Meanwhile, the infrastructure used to transport sewage is built on rapidly thawing and heaving ground. Pipes can rupture and spill, causing outbreaks of waterborne diseases.

"The ways people have dealt with human waste may not now be appropriate," Hueffer says.

The final report of the meeting in Hannover hasn’t been released yet. But the general consensus, according to Kutz, was that we don’t need to worry about a disease as contagious and deadly as COVID-19 coming out of the permafrost based on what’s been seen so far — but there are other reasons to be concerned.

The thawing permafrost may be home to bacteria and viruses we haven’t yet encountered — or, troublingly, ones that we have encountered with disastrous results, such as the Spanish flu or smallpox — but much of their DNA is in fragments, is adapted to infect other creatures or likely won’t come into contact with humans.

The key, Kutz says, will be to watch the wildlife, and that’s what she is doing: her lab works in collaboration with Indigenous harvesters across the Arctic to keep an eye on animal health.

"If you think about the wildlife, their noses are in the grass, they’re digging in the dirt," she says. "They’re the sentinels."