There could be many deadly diseases underground in permafrost. Permafrost is in mountains and near the North and South Poles. Climate change is melting it. 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 permafrost that are a million years old. A scientist said these "ancient viruses" could be terrifying. We have never seen them before. The scientist said deadly diseases like smallpox could be released. Factor X could be an even bigger problem. We need to learn more about Factor X.