By Ed Yong

For the world’s viruses, this is a time of unprecedented opportunity. An estimated 40,000 viruses lurk in the bodies of mammals, of which a quarter could conceivably infect humans. Most do not, because they have few chances to leap into our bodies. But those chances are growing. Earth’s changing climate is forcing animals to relocate to new habitats, in a bid to track their preferred environmental conditions. Species that have never coexisted will become neighbors, creating thousands of infectious meet-cutes in which viruses can spill over into unfamiliar hosts—and, eventually, into us. Many scientists have argued that climate change will make pandemics more likely, but a groundbreaking new analysis shows that this worrying future is already here, and will be difficult to address. The planetary network of viruses and wildlife “is rewiring itself right now,” Colin Carlson, a global-change biologist at Georgetown University, told me. And “while we thought we understood the rules of the game, again and again, reality sat us down and taught us: That’s not how biology works.”