The history of transmissible spongiform encephalopathies is shortly reviewed beginning with the Westminster parliament act in the year 1755 up to the description in 1996 of the variant of the Creutzfeldt-Jakob disease, transmitted from cattle to man by alimentary route. The epidemiological patterns of encephalopathies of the various animal species and of the four encephalopathies up to date reported in man are shortly described: Creutzfeldt-Jakob disease, Kuru, Gerstmann-Straussler-Scheinker disease, Fatal Familial Insomnia. Etiological hypotheses are discussed until the identification of Prions: PrPcell, on the surface of normal cells, PrPscr in the brain of humans and animals dead for these diseases. The strains of the PrPscr are described on the basis of some characters observed through the passages in rodents and of molecular pattern. The possible future epidemiological evolution of the vCJD is also discussed.