faction 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
constitution, a supposed peculiar condition of the body due to changes
in the character of the air, or to the climate, or to changes in the
interior of the earth as shown by earthquakes, or to the movements of
planets; in consequence of this peculiar constitution there was a
greater susceptibility to disease, but the direct cause might arise in
the interior of the body or enter the body from without. The character
of the disease which appeared in epidemic form, the "Genius
epidemicus," was determined not by differences in the intrinsic cause,
but by the type of constitution which prevailed at that time. The
first epidemic of cholera which visited Europe in 1830-37 was for the
most part referred to the existence of a peculiar epidemic
constitution for which various causes were assigned. It was only when
the second epidemic of this disease appeared in 1840 that the
existence of some special virus or poison which entered the body was
assumed.

Meanwhile, by the study of the material of disease knowledge was being
slowly acquired which had much bearing on the causes. The first
observations which tended to show that the causes were living were
made by a learned Jesuit, Athanasius, in 1659. He found in milk,
cheese, vinegar, decayed vegetables, and in the blood and secretions
of cases of plague bodies, which he described as tiny worms and which
he thought were due to putrefaction. He studied these objects with the
simple lenses in use at that time, and there is little doubt that he
did see certa