I, 12 (circa 40 B.C.), speaks of minute organisms which the eye cannot see and which enter the body and cause disease.
Out of these researches arose a famous battle which kept Pasteur hard at work for four or five years--the struggle over spontaneous generation.
It was an old warfare, but the microscope had revealed a new world, and the experiments on fermentation had lent great weight to the omne vivum ex ovo doctrine.

The famous Italians, Redi and Spallanzani, had led the way in their experiments, and the latter had reached the conclusion that there is no vegetable and no animal that has not its own germ.

But heterogenesis became the burning question, and Pouchet in France, and Bastian in England, led the opposition to Pasteur.

The many famous experiments carried conviction to the minds of scientific men, and destroyed forever the old belief in spontaneous generation.

All along, the analogy between disease and fermentation must have been in Pasteur's mind; and then came the suggestion, "What would be most desirable is to push those studies far enough to prepare the road for a serious research into the origin of various diseases." If the changes in lactic, alcoholic and butyric fermentations are due to minute living organisms, why should not the same tiny creatures make the changes which occur in the body in the putrid and suppurative diseases?
With an accurate training as a chemist, having been diverted in his studies upon fermentation into the realm of biology, and nourishing a strong conviction of the identity between putrefactive changes of the body and fermentation, Pasteur was well prepared to undertake investigations which had hitherto been confined to physicians alone.
So impressed was he with the analogy between fermentation and the infectious diseases that, in 1863, he assured the French Emperor of his ambition "to arrive at the knowledge of the causes of putrid and contagious diseases." After a study upon the diseases of wines, which has had most important practical bearings, an opportunity arose which changed the whole course of his career, and profoundly influenced the development of medical science.