A group of genetic, infectious, or sporadic degenerative human and animal nervous system disorders associated with abnormal prions. In humans, these conditions generally feature dementia, ataxia, and a fatal outcome. Diseases in this category include Creutzfeldt-Jakob Syndrome; scrapie; bovine spongiform encephalopathy; chronic wasting disease of mule deer and elk; and transmissible mink encephalopathy. Pathologic features include a spongiform encephalopathy without evidence of inflammation.