The hippocratic treatise Airs, waters and places is a book for travelling practitioners, explaining them all they have to check when they arrive in a new unknown place. According to the author of this manual, one of the most frequent causes of diseases is the bad quality of environmental air. Admitting this theory, the medical man could not imagine that diseases might be transmitted by contagion. Yet vets had always known about contagion among herds and flocks. Christian literature which used so frequently the metaphor of the good sheperd and the sick sheep might have drawn doctors' attention and parallelized the situation among animals and that among human beings. The authority of the so-called father of medicine was so powerful that this did not happen and that contagionists and anticontagionists would quarrel for ages.