[Prions: definition and diseases].

In this article we review the concept and terminology of prions, their replication and some current hypothesis on the nature of these infectious agents causing neurodegenerative diseases. This revision also summarizes the etiopathogenic, epidemiological, clinical and neuropathological features of the prion diseases or human transmissible spongiform encephalopathies, and some methods for their early diagnosis. Finally, we discuss the possible link between the bovine spongiform encephalopathy and the new cases of Creutzfeldt-Jakob disease identified in the United Kingdom.