In spite of a long history of intense investigation the transmissible spongiform encephalopathies remain a poorly understood family of neurodegenerative diseases. This group of diseases has been described in a wide variety of animal species and includes kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler syndrome in humans, and scrapie, bovine spongiform encephalopathy, and related syndromes in ruminants and rodents. In all cases spongiform degeneration and astrocytosis are seen in specimens of brain and a filterable transmissible agent is present in the brain and some other tissues of...