INFECTIOUS DISEASES. 623 



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 char- 

 acteristic, 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 exception- 

 ally 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, multi- 

 plies 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 re- 

 quirements 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 prod- 

 ucts by some substance present in the body of the animal which 

 survives infection. The composition of the body fluids, and es- 

 pecially 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 ex- 

 cessive 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. 



