Over the last decade, scientists have discovered the importance and widespread presence in the body of a remarkable family of proteins known as prion proteins. Research links various types of prion proteins to neurodegenerative diseases such as Parkinson's and Alzheimer's--and this has fueled the search for new drugs that could halt the progression of these terrible disorders. Other discoveries have revealed the essential roles prion proteins play in memory and immunity, and the part they may have played in the beginnings of life on our planet. Brahic, a leading researcher on diseases of the central nervous system, first describes the discovery of prions and their role in infection, beginning with early work on the animal disease scrapie and a mysterious human illness in New Guinea, apparently transmitted by cannibalism. Prion proteins were then revealed as the cause of other illnesses, from "mad cow" disease and its human counterpart, Creutzfeldt-Jakob disease, to such noninfectious brain disorders as Parkinson's and Alzheimer's. While the prion proteins responsible for diseases are definitively "bad," Brahic also explains that these abnormal prions are rare exceptions. Most of the time, prion proteins actually serve "good" and vital functions--and they may even have been present at the origin of life itself.