Book excerpt: A series of remarkable discoveries in the past three decades have led to the molecular and genetic characterization of the transmissible pathogen causing scrapie in animals and a quartet of human illnesses: kuru, Jakob-Creutzfeld disease, Gerstmann-Sträussler-Scheinker disease, and fatal familial insomnia. To distinguish this pathogen from viruses and viroids, the term "prion" was introduced to emphasize its proteinaceous and infectious nature. Stanley B. Prusiner, editor of this volume, was awarded the 1997 Nobel Prize in Physiology and Medicine for his pioneering discovery of prions. The book reviews advances in studies of prions, which - as considereable evidence indicates - are novel pathogens composed only of protein.