[Unconventional transmissible agents or prions]

Unconventional transmissible agents, or prions, induce rare, slow, neurodegenerative diseases that are transmissible and always fatal: transmissible subacute spongiform encephalopathies. These diseases are characterised by spongiosis associated with gliosis without inflammation or demyelination. These unconventional transmissible agents have particular biological and physiochemical properties that set them apart from the other micro-organisms. They are resistant to temperatures over 200 degrees C and insensitive to usual disinfectants used in virology; in addition, they are resistant to all the processes that degrade nucleic acids, while being sensitive to those which modify the structure or the composition of proteins. The only specific biological abnormality identified in the brain of affected patients is post-transcriptional accumulation of a normal host protein, PrP, in a form that makes it resistant to proteases; its accumulation is proportional to the titer of infection. The study of animal models in these diseases shows that the nervous system is by far the most infected organ. Infection is present in many organs long before the appearance of histological or clinical signs, explaining the accidental iatrogenic contamination thus far observed.