As a result of the conflict between the toxins and the defensive forces of the body, certain vital processes are set free in the blood and in the cells which seem to possess a highly specialized power of defense against any subsequent attack. Pasteur, in his researches on the subject of rabies, developed this power of resistance by inoculating into rabbits the rabies infection of a monkey. Monkey rabies is not a severe form and is scarcely felt by the ordinary rabbit, but if the infective material (usually part of the spinal cord) of the monkey-infected rabbit is transferred to a second rabbit, the disease becomes more severe; and if the disease is passed from animal to animal, it may be built up into as severe a form as desired, up to the maximum. Pasteur found that by inoculating an individual with a one-day rabbit, that is, with the weakest brand of infection killing a rabbit in one day, and the next day with a two-day rabbit, that the person could receive this two-day inoculation without discomfort or danger because of the greater antagonism acquired by the preceding inoculation. Continuing the inoculations for fourteen days and making the strength of the infection stronger each day, at the end of the period it was found that the fourteenth inoculation, strong enough to produce the disease and kill a fresh subject, had, on account of the preceding inoculations, produced ability to withstand or counteract the actual disease developing perhaps at the same time. Fortunately, in the case of this disease, the shortest period for its development is fifteen days, and often it is a month or more after the bite of the dog before the disease develops. By successive inoculation of increasing strength for fourteen days, the system will have acquired a habitude to the disease which prevents the normal effects. Diphtheria is prevented in much the same way, except that in this case horses are used, their blood being strengthened to resist the disease by successive inoculations of the diphtheria poison. It is probable that all the bacterial diseases which exert their influence through the transmission of toxins in the blood may be counteracted by the production of an antitoxin when once the method of building up this antitoxin has been learned. At present, rabies, tetanus, diphtheria, and cerebrospinal meningitis are the four diseases for which antitoxin is made commercially and generally used. For a great many years, scientists have labored without success to find an antitoxin for consumption, and within the last year extensive experiments have been made in the American army on the use of antitoxin for typhoid fever. _Natural immunity._