[Biology of the transmissible agent responsible for subacute spongiform encephalopathies].

Transmissible spongiform encephalopathies, also called prion diseases, are brain degenerative ailments. These diseases, that are either infectious, or genetic, or sporadic in origin, are characterised by the accumulation of a host protein, the prion protein. Although the disease mainly implicates the brain, the immune system is involved in the propagation of the infectious agent which is always associated with the prion protein. This protein presents a normal structure in healthy brain but changes its conformation in association with the disease. It has been possible to reproduce some characteristics of this conversion in vitro. The protein only hypothesis is grounded on this conformational change which is correctly simulated by physico-chemical models. This hypothesis can explain most of the characteristics of prion diseases but the existence of an other element associated with infection cannot be ruled out.