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A group of genetic, infectious, or sporadic degenerative human and animal nervous system disorders associated with abnormal PRIONS. These diseases are characterized by conversion of the normal prion protein to an abnormal configuration via a post-translational process. In humans, these conditions generally feature DEMENTIA, ataxia, and a fatal outcome. Diseases in this category include CREUTZFELDT-JAKOB SYNDROME; GERSTMANN-STRAUSSLER-SCHEINKER SYNDROME; KURU; SCRAPIE; fatal familial insomnia; bovine spongiform encephalopathy (see ENCEPHALOPATHY, BOVINE SPONGIFORM); transmissible mink encephalopathy; and chronic wasting disease of mule deer and elk. Pathologic features include a spongiform encephalopathy without evidence of inflammation. The literature occasionally refers to these as unconventional SLOW VIRUS DISEASES. (From Proc Natl Acad Sci USA 1998 Nov 10;95 (23):13363-83)