The struggle against virulent diseases was becoming more and more the capital question for Pasteur. He constantly recurred to the subject, not only in the laboratory, but in his home conversations, for he associated his family with all the preoccupations of his scientific life. Now that the oxygen of air appeared as a modifying influence on the development of a microbe in the body of animals, it seemed possible that there might be a general law applicable to every virus! What a benefit it would be if the vaccine of every virulent disease could thus be discovered! And in his thirst for research, considering that the scientific history of chicken-cholera was more advanced than that of variolic and vaccinal affections--the great fact of vaccination remaining isolated and unexplained--he hastened on his return to Paris (September, 1880) to press physicians on this special point--the relations between small-pox and vaccine. “From the point of view of physiological experimentation,” he said, “the identity of the variola virus with the vaccine virus has never been demonstrated.” When Jules Guérin--a born fighter, still desirous at the age of eighty to measure himself successfully with Pasteur--declared that “human vaccine is the product of animal variola (cow pox and horse pox) inoculated into man and humanised by its successive transmissions on man,” Pasteur answered ironically that he might as well say, “Vaccine is--vaccine.” Those who were accustomed to speak to Pasteur with absolute sincerity advised him not to let himself be dragged further into those discussions when his adversaries, taking words for ideas, drowned the debate in a flood of phrases. Of what good were such debates to science, since those who took the first place among veterinary surgeons, physicians and surgeons, loudly acknowledged the debt which science owned to Pasteur? Why be surprised that certain minds, deeply disturbed in their habits, their principles, their influence, should feel some difficulty, some anger even in abandoning their ideas? If it is painful to tenants to leave a house in which they have spent their youth, what must it be to break with one’s whole education? Pasteur, who allowed himself thus to be told that he lacked philosophical serenity, acknowledged this good advice with an affectionate smile. He promised to be calm; but when once in the room, his adversaries’ attacks, their prejudices and insinuations, enervated and irritated him. All his promises were forgotten.