The article deals with the history of the concepts of epidemic diseases and contagion. The idea that illnesses could be transmitted from animals to humans and from humans to humans is totally absent in ancient medical theories, but present in the works of historicians, veterinaries, architects. Which is the idea that brings ancient medicine to the refusal of the idea of contagion? And can a supposed 'medical authority' still be a real obstacle toward the comprehension of the ethiology of infectious diseases?