the action of offended gods. The priests and charlatans who sought to
excuse their inability to treat epidemics successfully were quick to
affirm supernatural causes. Hippocrates (400 B.C.), with whom medicine
may be said to begin, thought such diseases, even then called
epidemics, were caused by the air; he says, "When many individuals are
attacked by a disease at the same time, the cause must be sought in
some agent which is common to all, something which everyone uses, and
that is the air which must contain at this time something injurious."
Aristotle recognized that disease was often conveyed by contact, and
Varro (116-27 B.C.) advanced the idea that disease might be caused by
minute organisms. He says, "Certain minute organisms develop which the
eye cannot see, and which being disseminated in the air enter into the
body by means of the mouth and nostrils and give rise to serious
ailments." In spite of this hypothesis, which has proved to be
correct, the belief became general that epidemics were due to
putrefaction of the air brought about by decaying animal bodies, (this
explaining the frequent association of epidemics and wars,) by
emanations from swamps, by periods of unusual heat, etc.

With the continued study of epidemics the importance of contagion was
recognized; it was found that epidemics differed in character and in
the modes of extension. Some seemed to extend by contact with the
sick, and in others this seemed to play no part; it was further found
impossible in many cases to show evidence of air contamination, and
contamination of the air by putrefactive material did not always
produce disease. Most important was the recognition that single cases
of diseases which often occurred in epidemic form might be present and
no further extension follow; this led to the assumption in epidemics
of the existence of some condition in addition to the cause, and which
made the cause operative. In this way arose the theory of the epidemic