Inoculation against hydrophobia was rendered possible by the discovery of the fact that the rabies or hydrophobia virus is found in a pure condition, free from other microbes, in the nervous centers of animals. The material for inoculation is prepared from such nervous matter, the virulence of which is rendered _fixe_, as will be mentioned below. The cholera microbe, which was subsequently named _comma bacillus_, was discovered by Koch in 1883, in the intestinal contents of cholera patients. Two years later cholera broke out in Spain, and Dr. James Ferran, a Spanish physician, began inoculating men with living cultures of comma bacillus taken from patients attacked with the disease. The procedure in its essential features corresponded to the pre-Jenner method of inoculation. The failure to fix the strength of the virus used for treatment rendered the method subject to the same uncertainty as that which was connected with inoculation with smallpox virus taken direct from patients. It was impossible to predict the effect of the injections. Comma bacilli taken from cholera patients may, under cultivation, show themselves extremely virulent, or, on the contrary, extremely mild. There are specimens which, when injected into a Guinea pig, even in an insignificant dose, will prove fatal to it, and there are others which will appear harmless when given in a dose seventy times greater. The immediate effect, and the protection caused by the inoculation, must, of course, vary accordingly. The attempt made by Ferran caused great interest, and a number of scientific commissions were sent to Spain from different countries of Europe to study the results of his work. They could, however, come to no conclusion, and the treatment speedily lost its position. Only some seven years later a method was found of fixing the strength of the cholera virus. I was connected with this stage of the work, and it may perhaps present some interest to the reader to relate the way in which the problem was solved, and to show how gradual is the development of ideas by which results in laboratory investigation are arrived at. It has been mentioned already that the virulence of microbes changes under the influence of different agents in Nature--heat, light, chemicals. When a virus is first obtained from a patient or outside a patient its preceding history, its antecedents, the conditions under which it lived before, are extremely variable. Jenner’s method of cultivating the smallpox virus by transferring it from calf to calf secured for that virus uniform conditions of life, and its strength could thus be maintained unchanged for an indefinite length of time. Pasteur, in the preparation of hydrophobia vaccine, followed the same plan, and found in the successive inoculation from rabbit to rabbit a method of propagating the hydrophobia virus in a uniform condition. But attempts made to cultivate in a similar way the comma bacillus by transferring it from animal to animal failed.