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.
The most susceptible animals for the cholera microbe are Guinea pigs. There are two principal methods of ingrafting upon them the virus: Koch's method of administering it through the mouth and leaving it to develop in the intestines of the animal, and Pfeiffer's of injecting it, not into the intestines, but into the abdominal or peritoneal cavity, where the intestines are lodged, by introducing there the virus with a hypodermic needle not allowed to penetrate into the intestines themselves. But by neither of these methods could the microbe be cultivated in an unbroken series of animals, as it became gradually weakened and soon lost its power of affecting such animals. For the purpose in question, cultivation in the peritoneal cavity had the advantage that in a healthy individual the peritoneum is free from other microbes, whereas in the intestines there are always present a large number of micro-organisms which interfere in variable ways with the growth of the particular microbes.
But when one inoculates the peritoneal cavity of a Guinea pig with a dose of cholera microbes sufficient to cause a fatal disease, it is found, when the animal dies, that the microbes have died also. Thus, the attempt to ingraft the virus from a first animal to a subsequent one is checked at the very beginning. This initial difficulty was overcome by merely giving to the first animal a dose larger than was necessary to cause a fatal effect. The animal then succumbs more rapidly, and the microbes have no time to disappear. At the post-mortem examination there is found, in the peritoneal cavity, a small amount of exudate liquid which contains large numbers of those microbes alive. When, however, that exudate is injected into the peritoneal cavity of a second animal that animal does not succumb to the infection, or even if it succumbs one finds that the microbes have again disappeared in this second animal. By starting with % a still larger initial dose one may have three, perhaps four, successive animals affected by the virus, but it invariably ends by being weakened, and finally dies out.