Throughout history, humans have existed side-by-side with bacteria and viruses. From the bubonic plague to smallpox, we have evolved to resist them, and in response they have developed new ways of infecting us.
We have had antibiotics for almost a century, ever since Alexander Fleming discovered penicillin. In response, bacteria have responded by evolving antibiotic resistance. The battle is endless: because we spend so much time with pathogens, we sometimes develop a kind of natural stalemate.
However, what would happen if we were suddenly exposed to deadly bacteria and viruses that have been absent for thousands of years, or that we have never met before?
We may be about to find out. Climate change is melting permafrost soils that have been frozen for thousands of years, and as the soils melt they are releasing ancient viruses and bacteria that, having lain dormant, are springing back to life.
In August 2016, in a remote corner of Siberian tundra called the Yamal Peninsula in the Arctic Circle, a 12-year-old boy died and at least twenty people were hospitalised after being infected by anthrax.
The theory is that, over 75 years ago, a reindeer infected with anthrax died and its frozen carcass became trapped under a layer of frozen soil, known as permafrost. There it stayed until a heatwave in the summer of 2016, when the permafrost thawed.
This exposed the reindeer corpse and released infectious anthrax into nearby water and soil, and then into the food supply. More than 2,000 reindeer grazing nearby became infected, which then led to the small number of human cases.
The fear is that this will not be an isolated case.
As the Earth warms, more permafrost will melt. Under normal circumstances, superficial permafrost layers about 50cm deep melt every summer. But now global warming is gradually exposing older permafrost layers.
Frozen permafrost soil is the perfect place for bacteria to remain alive for very long periods of time, perhaps as long as a million years. That means melting ice could potentially open a Pandora's box of diseases.
The temperature in the Arctic Circle is rising quickly, about three times faster than in the rest of the world. As the ice and permafrost melt, other infectious agents may be released.
"Permafrost is a very good preserver of microbes and viruses, because it is cold, there is no oxygen, and it is dark," says evolutionary biologist Jean-Michel Claverie at Aix-Marseille University in France. "Pathogenic viruses that can infect humans or animals might be preserved in old permafrost layers, including some that have caused global epidemics in the past."
In the early 20th Century alone, more than a million reindeer died from anthrax. It is not easy to dig deep graves, so most of these carcasses are buried close to the surface, scattered among 7,000 burial grounds in northern Russia.
However, the big fear is what else is lurking beneath the frozen soil.
In a project that began in the 1990s, scientists from the State Research Center of Virology and Biotechnology in Novosibirsk have tested the remains of Stone Age people that had been found in southern Siberia, in the region of Gorny Altai. They have also tested samples from the corpses of men who had died during viral epidemics in the 19th Century and were buried in the Russian permafrost.
The researchers say they have found bodies with sores characteristic of the marks left by smallpox. While they did not find the smallpox virus itself, they have detected fragments of its DNA.
Certainly it is not the first time that bacteria frozen in ice have come back to life.