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It was recognized early on that the agent responsible for transmissible spongiform encephalopathies such as scrapie in sheep or kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker disease (GSS), and fatal familial insomnia (FFI) in man had quite extraordinary properties, such as unusually long incubation periods (measured in months to years) and uncommon resistance to high temperature, formaldehyde treatment, and UV irradiation. The agent has been designated as “prion” (see glossary, Table 1) to distinguish it from conventional pathogens such as bacteria and viruses (Prusiner 1982). In recent years, a new form of prion disease emerged in Great Britain and to a lesser extent in other European countries, namely, bovine spongiform encephalopathy (BSE) or mad cow disease, which has been attributed to the consumption by cattle of feed supplements derived from scrapie-contaminated sheep and later from cattle offal (Wilesmith et al. 1992). It is, however, quite possible that BSE originated as a sporadic.