[The biology of prions, a response to the enigma of spongiform encephalopathies of sheep and humans].

Prions are small proteinaceous infectious particles without nucleic acids. The prion protein arise from a cellular protein by a post-transduction event and constitute amyloid deposits. Prions are the agent of animal and human spongiform encephalopathies which evolve on simultaneous infectious and genetic dependence in the absence of a detectable immune response.