INDIVIDUAL VARIATIONS IN SUSCEPTIBILITY 133 



organisms will under ordinary conditions produce no deleterious 

 results. If, however, the animals are kept at a temperature at which 

 anthrax bacilli can readily grow, infection promptly takes place. 

 Conversely, Pasteur found that by refrigeration it is possible to 

 infect chickens with anthrax, whereas normally the animal is 

 immune. 



Toward the leprosy bacillus there is apparently an absolute 

 immunity not only on the part of the cold-blooded animals, but 

 also of the vertebrates, with the exception of man and possibly 

 of certain monkeys. 



Generic Immunity. As examples of generic immunity we may 

 mention the resistance of man to the common organisms which are 

 pathogenic for the lower vertebrates, and vice versa. 



Species Immunity. Species immunity is illustrated by the resist- 

 ance of dogs, pigs, and rats to the anthrax bacillus, while cattle, 

 sheep, and most of the common laboratory animals are quite sus- 

 ceptible to infection with this organism. Cattle plague (rinderpest), 

 swine plague (Schweinerotlauf), sympathetic anthrax (Rauschbrand), 

 chicken cholera, etc., do not affect man under normal conditions 

 while animals are naturally immune to infection with the cholera 

 vibrio, the meningococcus, the typhoid bacillus, the gonococcus, 

 the Treponema pallidum, as well as to such diseases as scarlatina, 

 measles, yellow fever, poliomyelitis, etc. 



Racial Immunity. Racial immunity is exemplified by the relatively 

 high degree of resistance of Algerian sheep to anthrax, to which 

 our own domestic sheep are very prone. Black rats are more resist- 

 ant to anthrax then gray rats and gray rats more so than white 

 rats. The same point is also well shown in the remarkable difference 

 in the susceptibility of different races to such diseases as measles, 

 smallpox, tuberculosis, etc. 



Individual Variations in Susceptibility. The occurrence of indi- 

 vidual variations in the susceptibility to various diseases, further, 

 is so well known as hardly to require special mention. During 

 epidemics of cholera, smallpox, diphtheria, yellow fever, typhoid 

 fever, influenza, etc., this is particularly noticeable. There are then 

 always some persons who escape infection even though they have 

 been freely exposed, and among those which develop the diseases 

 in question there are some in whom the malady runs a mild course, 

 while otheres are fatally stricken; in some we see a remarkable 



