Prions are an entirely new class of pathogens, and scientists are just beginning to understand them. Although they have plagued humans and animals for 3 centuries, only in the last 2 decades have researchers linked them to diseases like Mad Cow. This book tells the strange story of their discovery, and the medical controversies that swirl around them. The author, Philip Yam, is a well-respected and connected journalist who is now an editor at Scientific American.