The nature of the unconventional slow infection agents remains a puzzle.

Unconventional slow infections are progressive transmissible degenerative disorders of the central nervous system. The human diseases belonging to this group are Creutzfeld-Jakob disease, kuru, and Gerstmann-Straussler syndrome. Scrapie, transmissible mink encephalopathy, chronic wasting disease of mule deer and elk, and the recently discovered bovine spongiform encephalopathy are similar diseases found in animals. Unusual characteristics of the unconventional slow infections clearly distinguish these disorders from conventional infections. These include: unusually long incubation periods (from months to years); progressive CNS degeneration with characteristic histopathological lesions; the lack of an immune or inflammatory response; unconventional biological and physical properties of the etiologic agents. There has been considerable controversy concerning the nature of the causative agent. The 3 main hypotheses, virus, virino, and modified host protein, are reviewed relative to their ability to explain the properties of the agent and the unusual characteristics of the disease process. The discovery of an abnormal structure, termed scrapie associated fibrils (SAF) and an abnormally modified 33-37 kDa host-encoded glycoprotein unique to unconventional slow infections opened new areas of intense interest and investigation. SAF are abnormal filamentous structures which copurify with infectivity and possess characteristics of "amyloids." The major component of SAF is the host-encoded scrapie-specific protease resistant glyco-protein. Considerable data has accumulated on the biochemistry, immunology and molecular biology of this host coded scrapie protein. The relationship of SAF and the scrapie-specific protein to the infectious agent is discussed in the context of each of the "nature of the agent" hypotheses.