The observation that a relevant part of a population is simultaneously sick is quite ancient. Nevertheless, the explanation and characterization of such phenomenon, namely at the medical framework, had changed throughout the time. Hippocrates, the most influential doctor in European medicine until the end of the 18th century, described, under the name of epidemy, diverse endemic situations, and attributed them to the peculiar “constitution” of the place affected in a certain moment. But he did not approach specifically the phenomenon of the transmission of a disease by means of the contact of healthy persons with someone affected by a disease. So, the first description of an epidemy, by the historian Thucydides, has no parallel in the medical writings of his time. In any case, it motivated the retrospective effort of relevant doctors, as Galen, aiming to explain the phenomenon of disease’s transmission. The theory of contagion is relatively recent, being formulated by Girolamo Fracastoro in his De contagione (1546). This Physician and humanist received the word contagion from the intellectual Latin tradition and converted it into a central concept of medical science and practice.