The study of that group of diseases now collectively known as the prion diseases has always been a source of excitement and argument between scientists. These obscure diseases usually have been of extremely rare occurrence and have had little impact on the general public. When an epidemic occurs, however, as in the case of bovine spongiform encephalopathy (BSE) in the United Kingdom, the bizarreness of the prion diseases and the profound difference between them and any other “infectious” condition can (and did) lead to public consternation. Three features of prion diseases give them an apparently diabolical quality.