The study also raised concerns about the thawing of permafrost due to climate change. The researchers found that the viruses extracted from melting icy cold surfaces in Siberia are all genomically distinct from all known existing viruses.

The oldest of these "zombie viruses" has been identified as Pandoravirus yedoma, a virus that has set a record for the longest life span among frozen viruses (48,500 years), capable of returning to an infectious state. Early in the isolation process, the virus was visible under a light microscope. The virus called Cedratvirus was recovered from the Russian Lena River, the Kamchatka Peninsula and the mud flowing into the Kolyma River. A sample of Pithoviruses was even collected from a large amount of mammoth wool.

Although Pacmanviruses have recently been linked with a few cases of African swine fever, scientists have now reported a new version of the virus found in the frozen intestinal remains of a Siberian wolf that is 27,000 years old.

If the permafrost continues to melt as a result of increased temperatures due to climate change, it will be necessary to determine how infectious these viruses may become once they are re-exposed to light, heat and oxygen. According to the scientists, the biological risk of reviving the viruses was "negligible" because of the strains they were targeting, which are primarily capable of infecting amoeba microbes.

But the potential and possible resurgence of a virus that can infect other organisms (animals or even humans) could be a real threat. The risk is certain to increase with global warming as the melting of permafrost continues to accelerate and more people populate the Arctic in the wake of industrial enterprise. Thus, it is likely that thawing of ancient permafrost releases these unknown viruses. It is currently impossible to estimate how long these viruses can remain infectious when exposed to outdoor conditions, and how likely they are to encounter and infect a suitable host during this time interval.