In March 1996, the British Secretary of State for Health announced that 'Mad Cow Disease' (BSE) had transmitted to humans. One striking aspect of this new episode of the BSE crisis was the recurrent appearance of 'prions', thought to be the infectious agents of this disease. The history of prions displays several interesting characteristics. Prions are enigmatic agents usually described as 'infectious proteins', whose pathogenecity is not yet fully understood. If the prion hypothesis, which holds that BSE and similar diseases are caused by such agents, is now widely accepted by the scientific community, many researchers are still disputing it. In the 1980s, Stanley Pruisner, the proponent of the prion hypothesis, was regarded as an heretic. In 1997, however, he won the Nobel Prize for medicine for his discovery of prions. Ths book sheds light on such paradoxical features and is the first to look at the prion case from an historical and social perspective.