Considerable differences as to susceptibility may also exist among adults of the same species. In man these differences in individual susceptibility to infectious diseases are frequently manifested. Of a number of persons exposed to infection in the same way, some may escape entirely, while others have attacks differing in severity and duration. In our experiments upon the lower animals we constantly meet with similar results, some individuals proving to be exceptionally resistant. Exceptional susceptibility or immunity may be to some extent a family characteristic, or one of race. Thus, the negro race is decidedly less subject to yellow fever than the white race, and this disease is more fatal among the fair-skinned races of the north of Europe than among the Latin races living in tropical or subtropical regions. On the other hand, small-pox appears to be exceptionally fatal among negroes and dark-skinned races generally. A very remarkable instance of race immunity is that of Algerian sheep against anthrax, a disease which is very fatal to other sheep.
The essential difference between a susceptible and immune animal depends upon the fact that in one the pathogenic germ, when introduced by accident or experimental inoculation, multiplies and invades the tissues or the blood, where, by reason of its nutritive requirements and toxic products, it produces changes in the tissues and fluids of the body inconsistent with the vital requirements of the infected animal; while in the immune animal multiplication does not occur or is restricted to a local invasion of limited extent, and in which after a time the resources of Nature suffice to destroy the parasitic invader.
Now, the question is, Upon what does this essential difference depend? Evidently upon conditions favorable or unfavorable to the development of the pathogenic germ; or upon its destruction by some active agent present in the tissues or fluids of the body of the immune animal; or upon a neutralization of its toxic products by some substance present in the body of the animal which survives infection. The composition of the body fluids, and especially their reaction, is probably a determining factor in some instances. Thus, Behring has ascribed the failure of the anthrax bacillus to develop in the white rat, which possesses a remarkable immunity against anthrax, to the highly alkaline reaction of the blood and tissue juices of this animal. Behring claims to have obtained experimental proof of the truth of this explanation by feeding white rats on an exclusive vegetable diet, or by adding acid phosphate of lime to their food, by which means this excessive alkalinity of the blood is diminished. Hats so treated are said to lose their natural immunity, and to die as a result of inoculation with virulent cultures of the anthrax bacillus.