The pithovirus sibericum was unleashed from a 100ft deep layer of the Siberian permafrost. Though this virus poses no danger to humans or animals, the discovery has raised fears other more deadly pathogens could be released from the frozen ground. If it is true that these viruses survive in the same way those amoeba viruses survive, then smallpox is not eradicated from the planet - only the surface, Dr Chantal Abergel, a co-author of the report published in the Proceedings of the National Academy of Sciences said. "By going deeper we may reactivate the possibility that smallpox could become again a disease of humans in modern times."
However, it is not yet clear whether all viruses could become active again after being frozen for thousands or even millions of years.