In investigating chicken cholera Pasteur discovered another great basic principle in the knowledge of disease, especially of its treatment. After considerable difficulty he succeeded in finding the germ of this disease which was causing great losses in the poultry industry of France and other European countries. This germ was cultivated for a number of generations on artificial media and never failed to produce the disease when fowls had been inoculated. During the course of his studies in the malady Pasteur was called away to a distant part of France in connection with his investigation of anthrax. He was away from his laboratory for several months. When he returned he inoculated {310} some fowls with the cultures of chicken cholera which he had left behind. To his surprise and annoyance the inoculations failed to produce the typical symptoms of the disease. The fowls suffered from some slight symptoms and then recovered. When he left his laboratory inoculations had been invariably fatal. It took considerable time and trouble to procure fresh cultures of the chicken-cholera microbe. Meantime, the fowls which had been only slightly affected by the old cultures were carefully preserved. When these birds were inoculated with the fresh virulent cultures they failed to take the disease. Other fowls promptly died, exhibiting all the characteristic symptoms of chicken cholera. Those that had suffered from the mild form of the disease produced by the old cultures were protected from further attacks of the disease. One of the great mysteries of medicine, the varying virulence of disease, had been thus solved by what seemed an accident. There are no accidents in the lives of great investigators. There are surprises, but genius knows how to reconcile their occurrence with the principles they are working out. Pasteur understood at once the wonderful utility there might be in this discovery for the protection of men and animals from disease. He proceeded to practical applications of the new theory by providing old cultures for the inoculation of fowls in districts where chicken cholera produced serious ravages. Then, working on the same lines as for chicken cholera, he proceeded to elaborate vaccine material for anthrax. Vaccine was the name deliberately selected for the inoculating substance in order to honor the genius of the English physician Jenner, who had discovered the power of vaccination to protect from smallpox. The weakening of the germs of anthrax, so as to produce only a mild form of the disease, was a much more intricate problem than for chicken cholera, {311} because the anthrax bacillus does not weaken with age, but enters a resting or spore stage, resembling the seed stage in large plants. After a patient series of investigations Pasteur accomplished his object by some ingenious methods that served to show, perhaps better than any other details of his career, how thoroughly practical was his inventive genius.