Ebola, SARS, Hendra, AIDS, and countless other deadly viruses all have one thing in common: the bugs that transmit these diseases all originate in wild animals and pass to humans by a process called spillover. In this gripping account, David Quammen takes the reader along on this astonishing quest to learn how, where from, and why these diseases emerge and asks the terrifying question: What might the next big one be?

Diseases that were contained are being set free and the results are potentially catastrophic. In a journey that takes him from southern China to the Congo, from Bangladesh to Australia, David Quammen tracks these infections to their source, and asks what we can do to prevent some new pandemic spreading across the face of the earth.