As bad as this year’s flu epidemic has been, at least in the United States, most of us rest easy knowing that deadlier and nastier mass-murdering pathogens such as bubonic plague and smallpox live mostly in our collective memory. Or do they? As climate change melts longstanding permafrost, some scientists fear that “zombie pathogens,” which have been slumbering for centuries, might be waking up, threatening to overtake humanity again.

Permafrost refers to a layer of permanently frozen earth—it has to be frozen for a minimum of two years to qualify—found primarily in most continually frosty parts of the world such as the Arctic Circle, Greenland, Alaska and Siberia. According to National Geographic, there are some 22.8 million square miles of permafrost in the world. Research has shown that Earth's permafrost heated up by 6 degrees Celsius during the 20th century and scientists predict even more dramatic melting by 2100. Not only will this raise ocean levels and exacerbate erosion, it may also mean a release of pathogens better left frozen.