The emergence of strange new diseases—Ebola, SARS, Hendra—is a frightening problem that seems to be getting worse: In this age of speedy travel, it augurs a worldwide pandemic. Even scarier, says science writer David Quammen: Such phenomena are part of a single pattern, as the bugs that transmit these diseases all originate in wild animals and pass to humans by a process called spillover. His book, appropriately titled Spillover, tracks this subject around the world to learn how, where from, and why these diseases emerge, and then asks: What might the next big one be?