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.