Old bacteria under the ground could contain many deadly diseases. The bacteria have been trapped in permafrost for thousands of years. Permafrost is long-frozen ground. It is common in mountains and near the North and South Poles. It is melting because of climate change. This could release unknown, harmful viruses. Scientists are calling these 'Factor X'. An infectious diseases professor said we know little about Factor X.

There are viruses in the permafrost that are up to a million years old. A virologist said these unknown, "ancient viruses" could be terrifying because we have never seen them before. He also said deadly diseases like smallpox could be released worldwide. Factor X could be an even bigger problem. However, scientists say the release of methane is a bigger danger from melting permafrost. Methane is 80 times more harmful than CO2.