228 GENERAL BIOLOGY OF MICRO-ORGANISMS 



with difference in food habits. Thus the carnivorous mammals 

 are relatively insusceptible to anthrax and tuberculosis, diseases 

 natural to the herbivora. Many infectious diseases of man 

 are not readily transmissible to anitnals, for example, typhoid 

 fever, syphilis, pneumonia, and in some instances it has so far 

 been impossible to infect animals, as for example with scarlet 

 fever and gonorrhea. "^ 



Racial Immunity.- — Within a species there is moreover a 

 racial difference in resistance to natural infection. Thus the 

 pure-bred dairy cattle are more susceptible to tuberculosis than 

 other cattle, -and Yorkshire swine are relatively less susceptible 

 to swine erysipelas. In man, the relation of race to susceptibility 

 is not very clear. The examples of supposed racial immunity 

 have not proved to be so definite as has been assumed at first. 

 Thus the supposed immunity of African natives to syphiUs has 

 vanished with their increasing contact with civilization and 

 with this accompanying disease. Iii the case of malaria the 

 supposed racial immunity of negroes seems to be an acquired 

 immunity due to severe attacks of the disease in childhood. 

 There is, however, some evidence that prolonged contact with a 

 disease through many generations may result in a relative resist- 

 ance, so that the disease assumes a milder form in such a race of 

 people — a sort of inherited acquired immunity. Such considera- 

 tions have been brought forward to explain the relatively high 

 resistance to tuberculosis shown by the Hebrews as compared 

 with the American Indians. \ 



Individual Variations. — Individual variations in resistance 

 to infection are commonly observed. They may depend in part 

 upon age, condition of nutrition, fatigue, exposure or intoxica- 

 tion, but they are ascribed also to differences in anatomical 

 structure (shape of the thorax in tuberculosis). Individuals 

 especially susceptible to a disease are said to possess an idiosyn- 

 crasy for it. The physiological mechanisms upon which varia- 

 tions in individual resistance depend are not clearly understood. 



1 KoUe und Wassermann, II Auflage, Bd. IV, p. 693 (1912). 



