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.