At the middle of the last century we did not know much more of the actual causes of the great scourges of the race, the plagues, the fevers and the pestilences, than did the Greeks. Here comes Pasteur's great work. Before him Egyptian darkness; with his advent a light that brightens more and more as the years give us ever fuller knowledge. The facts that fevers were catching, that epidemics spread, that infection could remain attached to articles of clothing, etc., all gave support to the view that the actual cause was something alive, a contagium vivum. It was really a very old view, the germs of which may be found in the Fathers, but which was first clearly expressed--so far as I know--by Fracastorius, the Veronese physician, in the sixteenth century, who spoke of the seeds of contagion passing from one person to another;(12) and he first drew a parallel between the processes of contagion and the fermentation of wine. This was more than one hundred years before Kircher, Leeuwenhoek and others began to use the microscope and to see animalcula, etc., in water, and so give a basis for the "infinitely little" view of the nature of disease germs. And it was a study of the processes of fermentation that led Pasteur to the sure ground on which we now stand. (12) Varro, in De Re Rustica, Bk. 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