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 certain of the larger organisms which are present in vinegar, cheese and decaying vegetables, and it is not impossible that he may have seen the animal and vegetable cells.

The first description of bacteria with illustrations showing their forms was given by Loewenhoeck, a linen dealer in Amsterdam in 1675.  The fineness of the linen being determined by the number of threads in a given area, it is necessary to examine it with a magnifying lens, and he succeeded in perfecting a simple lens with which objects smaller than had been seen up to that time became visible.  It must be added that he was probably endowed with very unusual acuteness of vision.  He found in a drop of water, in the fluid in the intestines of frogs and birds,