Transmissible spongiform encephalopaties (TSE) are a group of rare fatal diseases of humans and animals. Prions, small infectious proteinaceous particles, are their supposed cause. Prusiner's theory (Nobel price 1997) proves that pathogenetically active prions are conformated physiological prions but problems of pathogenesis of TSE are still open. Most important representative of human TSE is Creutzfeldt-Jakob's disease (CJD), that of animal TES is bovine spongiform encephalopathy (BSE). It seems that a new variant of CJD found in more than 14 young people in the UK is in reality human BSE. Whether it means start of an epidemy of human BSE is not known.