There’s a working theory for the origins of Covid-19. It goes like this: Somewhere in an open-air market in Wuhan, China, a new coronavirus, growing inside an animal, first made the jump to a human. But what happens when diseases spread in the other direction? Sonia Shah, a science journalist, explores the dangers of “spillback,” or “reverse zoonosis”: when humans infect non-humans with disease. Using the history of diseases spreading through mink farms in the United States and Europe as a focus, Shah considers the implications of spillback, and how we might minimize its future impact. Shah considers how spillback can ignite epidemics in wild species, including endangered ones, and can ravage whole ecosystems. More worryingly, she describes how it can establish new wildlife reservoirs that shift the pathogens’ evolutionary trajectory, unleashing novel variants that can fuel new, dangerous waves of disease in humans.