This past summer anthrax killed a 12-year-old boy in a remote part of Siberia. At least 20 other people, also from the Yamal Peninsula, were diagnosed with the potentially deadly disease after approximately 100 suspected cases were hospitalized. Additionally, more than 2,300 reindeer in the area died from the infection. The likely cause? Thawing permafrost. According to Russian officials, thawed permafrost—a permanently frozen layer of soil—released previously immobile spores of Bacillus anthracis into nearby water and soil and then into the food supply. The outbreak was the region's first in 75 years.

Researchers have predicted for years that one of the effects of global warming could be that whatever is frozen in permafrost—such as ancient bacteria—might be released as temperatures climb. This could include infectious agents humans might not be prepared for, or have immunity to, the scientists said. Now they are witnessing the theoretical turning into reality: infectious microorganisms emerging from a deep freeze.

Although anthrax occurs naturally in all soil and outbreaks unrelated to permafrost can occur, extensive permafrost thaw could increase the number of people exposed to anthrax bacteria. In a 2011 paper published in Global Health Action, co-authors Boris A. Revich and Marina A. Podolnaya wrote of their predictions: “As a consequence of permafrost melting, the vectors of deadly infections of the 18th and 19th centuries may come back, especially near the cemeteries where the victims of these infections were buried.”

And permafrost is indeed thawing—at higher latitudes and to greater depths than ever before. In various parts of Siberia the active layer above permafrost can thaw to a depth of 50 centimeters every summer. This summer, however, there was a heat wave in the region, and temperatures hovered around 35 degrees Celsius—25 degrees warmer than usual. The difference possibly expanded or deepened the thaw and mobilized microorganisms usually stuck in rigid earth. Although scientists have yet to calculate the final depth, they postulate that it is a number that has not been seen in almost a century. Permafrost thaw overall could become widespread with temperatures only slightly higher than those at present, according to a 2013 study in Science. Heat waves in higher latitudes are becoming more frequent as well.

What thawing permafrost could unleash depends on the heartiness of the infectious agent involved. A lot of microorganisms cannot survive in extreme cold, but some can withstand it for many years. “B. anthracis are special because they are sporulating bacteria,” says Jean-Michel Claverie, head of the Mediterranean Institute of Microbiology and a professor at Aix-Marseille University in France. “Spores are extremely resistant and, like seeds, can survive for longer than a century.”

Viruses could also survive for lengthy periods. In 2014 and 2015 Claverie and his colleague Chantal Abergel published their findings on two still infectious viruses from a chunk of 30,000-year-old Siberian permafrost. Although Pithovirus sibericum and Mollivirus sibericum can infect only amoebas, the discovery is an indication that viruses that infect humans—such as smallpox and the Spanish flu—could potentially be preserved in permafrost.

Human viruses from even further back could also make a showing. For instance, the microorganisms living on and within the early humans who populated the Arctic could still be frozen in the soil. “There are hints that Neandertals and Denisovans could have settled in northern Siberia [and] were plagued by various viral diseases, some of which we know, like smallpox, and some others that might have disappeared,” Claverie says. “The fact that there might be an infection continuity between us and ancient hominins is fascinating—and might be worrying.”

Janet Jansson, who studies permafrost at the Pacific Northwest National Laboratory in Washington State, is not worried about ancient viruses. Several attempts to discover these infectious agents in corpses have come up empty, she notes. She does advocate, however, for further research to identify the wide range of permafrost-dwelling organisms, some of which could pose health risks. To accomplish that goal, she and others are using modern molecular tools—such as DNA sequencing and protein analysis—to categorize the properties of unknown microorganisms, sometimes referred to as microbial dark matter.

The likelihood and frequency of outbreaks similar to the one in Siberia will depend on the speed and trajectory of climate change. For instance, it is possible that another heat wave will expose the carcasses of animals infected by anthrax, Revich says. “The situation on the Yamal Peninsula has shown that the risk of the spread of anthrax is already real,” he adds.

In effect, infectious agents buried in the permafrost are unknowable and unpredictable in their timing and ferocity. Thus, researchers say thawing permafrost is not our biggest worry when it comes to infectious diseases and global warming. The more immediate, and certain, threat to humans is the widening geographical ranges of modern infectious diseases (and their carriers, such as mosquitoes) as the earth warms. “We now have dengue in southern parts of Texas,” says George C. Stewart, McKee Professor of Microbial Pathogenesis and chair of the department of veterinary pathobiology at the University of Missouri. “Malaria is seen at higher elevations and latitudes as temperatures climb. And the cholera agent, Vibrio cholerae, replicates better at higher temperatures.”

Unlike the zombie microbes lurking in the permafrost, modern spreading diseases are more of a known quantity, and there are proved ways to curb them: mapping trends, eliminating mosquito-breeding sites and spraying insecticides. Of course, dramatically lowering fossil-fuel emissions to combat climate change could tackle both threats—the resurgence of ancient and deadly pathogens and the widening ranges of infectious diseases—in one shot.