Jenner's vaccine was a transmitted cowpox did not militate against the general theory of protecting the individual against a severe form of a disease by the production of a mild form, for cowpox was generally considered to be smallpox modified by passage through another host, the bovine animal. If such results could be obtained against a disease, small-pox, the causal agent of which was unknown, how much easier to vaccinate against a disease of known etiology!
This was therefore the first line of attack in the battle for a specific therapy of the infectious diseases. Already Pasteur was at work. An epidemic of chicken-cholera, in 1880, offered the opportunity for extended experiments. In the course of this work, a chance observation gave him the clue to vaccination with bacteria of attenuated virulence. It had been his routine practise in the experimental production of chicken cholera to use fresh 24-hour cultures; these always produced the disease readily. But in the course of the work it happened that an old culture which had been set aside for a few weeks and forgotten, was used, with the unexpected result that the inoculated hens, although ill for a while, promptly recovered, and what was more surprising, remained refractory to subsequent inoculation of fresh cultures, though the same cultures were virulent for untreated hens. This phenomenon, the attenuation of virulence clue to artificial cultivation, Pasteur used as the basis of a treatment by vaccination, which had the immediate effect (1880) of reducing the mortality of chicken cholera to one per cent, and the more remote but far more important effect of stimulating the study of specific therapy. Incidentally it was the link between Lady Mary Wortley Montagu's preventive inoculation and Jenner's vaccination, on the one hand, and modern theories of the production of immunity on the other.
The next step was with anthrax, a disease, of cattle. The attenuation of chicken cholera virus had been due to artificial cultivation, but about this time Toussaint, of the veterinary school of Toulouse, made some observation on the attenuation of anthrax bacilli under the influence of increased temperature (heating to 55° C. for ten minutes). His observations, however, were without constant results. Pasteur, who was familiar with Toussaint's work, took up the matter and after a thorough investigation found that anthrax bacilli cultivated at a temperature of 42° to 43° C, became attenuated, and this attenuation persisted on artificial cultivation (1881). The inoculation of such organisms did not cause anthrax, and when later virulent bacilli of anthrax were inoculated, the animals were found to be immune. This was the scientific basis of the celebrated public test at Melun. Sixty sheep and ten cows were placed at the disposal of Pasteur; twenty-five of the sheep and six of the cows were to be vaccinated with attenuated anthrax bacilli, and after an interval of twelve to fifteen days this was to be