P. Rayer (1795-1867) had never thoroughly published his experimental studies on the contagion of glanders. His recently un-earthed hand written papers allow us to depict his experimental approach and its results. He was not the first who transmitted glanders from a patient to horses or donkeys. But he did it systematically with glander secretions from acute and chronic cases. Whatever was the disease of the donors the transmitted forms were unpredictably either chronic or acute. His conclusion was that the two forms were two symptomatic aspects of a unique disease. Clinically dormant states were shown to be also contagious. He demonstrated it through deliberately altering healthy and sick horses inside the stable and by using saddles, bridles and brushes of sick horses on healthy ones. Moreover he excluded other causative factors tentatively proposed, peculiarly food products. The systematically logical and rigorous experimental approach used by Rayer for this research is a mile stone, 30 years before Pasteur. This methodology is still nowadays used to study the epidemiology of diseases such as Prion Diseases, Mad Cow for instance.