[The truth and present uncertainty about mad cow disease].

A historical review is made about Spongiform Encephalopathies which affect both animals and man. This is the base for an epidemiological and predictive analysis of these type of diseases, especially Bovine Spongiform Encephalopathy (BSE) as a present health problem. The scientific certainties or truths, such as the prion theory (PrPc-PrPsc), the low natural infectivity of these group of diseases, the high dose of prions necessary to produce the experimental disease, the species barrier or specificity, the individual susceptibility due to genetic traits, and the low transmission efficiency by the oral route, compared to the parenteral route, agree with the epidemiological observations of human cases of the variant of the Creutzfeldt-Jakob disease (vCJD), which is 0.1 cases per million inhabitants and year. The present and future prediction of BSE should not be alarmist, taking into account the certainties that we know.