We have seen how the facts have been established with regard to the microbe of fowl cholera. Immunity against a virulent disease may be obtained by the influence of a benign malady which is induced by the same microbe, only weakened in virulence. What a future there would be for medicine if this method could be applied to the prevention of all virulent diseases! As splenic fever was at that time being studied in the laboratory of the École Normale, it was upon this fever that the research was first attempted. But the success of this research, said Pasteur, can only be hoped for if the disease is non-recurrent. It is only in this case that inoculation with the weakened microbe can protect from the deadly splenic fever. Unfortunately, human medicine is dumb as regards this question of non-recurrence. The man who is smitten with malignant pustule rarely recovers. If there are any cases of recovery--and there are some authentic ones--he who has so narrowly escaped death could not confidently count upon his chance of protection from the disease in future. In order to acquire such a sense of security he would have to expose himself to experiments of direct inoculation, which he would hardly care to do. Animals alone offer the possibility of solving this problem. Yet it is not to all species of animals that we can have recourse. Every sheep inoculated with splenic fever infection is a sheep lost; but the ox and the cow have more power of resistance. Among them there are frequent cases of cure. An incident occurred which enabled Pasteur to push very far this experimental study.