After initial skepticism, it is now generally accepted that prions exist, that they differ from all other pathogens, and that they are infectious in several species. Prions are implicated in spongiform encephalopathies such as kuru, Creutzfeldt-Jakob disease and fatal familial insomnia in humans, scrapie in sheep and a bovine encephalopathy that may be transmissible to humans. Driven by prions' novelty and by concern about their public health effects,...