Long-tailed macaques are passing on their own diseases to humans. (Photo by Andy Lawson/CCBY-NC 2.0) It can be tempting to think of health and the environment as two separate categories. One concerns what happens in hospitals and labs, where patients lie hooked up to blinking machines and scientists stare through microscopes at pathogens. The other concerns what happens out in the world, in felled forests and wildlife preserves. In his engaging new book Spillover: Animal Infections and the Next Human Pandemic, environmental writer David Quammen blows any such notions out of the water. Spillover is about infectious diseases, called zoonoses, that cross over from animals into humans. These include Ebola, SARS, and HIV, but Quammen takes readers on a tour of many other equally chilling emerging diseases. The book’s central question is, “What’s next?” Quammen doesn’t just look at what pathogens are cropping up. He also looks at why they’re emerging or re-emerging. He...