The discovery of an infectious giant virus that had been entombed in Siberian permafrost for 30,000 years has led scientists to warn of other disease-causing viruses and microbes that may escape from the frozen earth once it has melted.

Scientists in France and Russia discovered the giant virus in samples of frozen earth taken from the far north-east of Russia. Tests in the laboratory showed that the virus was capable of infecting amoeba – single-celled micro-organisms – although it cannot infect multi-cellular animals and humans.

The virus is much larger than usual viruses and is so big it can be seen under ordinary optical microscopes. It is similar to two other known types of giant viruses, but its genetic material is different enough for it to be classified as belonging to a distinct species, Pithovirus sibericum, within a totally new group of viruses.

Researchers at the Russian Academy of Sciences in Pushchino and France’s National Centre for Scientific Research (CNRS) in Marseille said that the virus was buried 30 metres (100 feet) below ground in the Chukotka autonomous region of Siberia and must have been frozen for at least 30,000 years before it burst back into life when offered the “bait” of living amoeba in a laboratory experiment.

“This study demonstrates that viruses can survive in permafrost – the permanently frozen layer of soil found in the Arctic regions – almost over geological time periods, that is for more than 30,000 years,” a CNRS spokeswoman said.

The lead researchers in Russia were Lyubov Shmakova and Elizaveta Rivkina of the Institute of Physicochemical and Biological Problems in Soil Science in Pushchino.