INFECTION 115 



in environment may predispose to infection; the carnivora in their 

 native state are quite resistant to infection with the tubercle bacillus, 

 whereas in captivity they may succumb readily. Similarly, man 

 placed in bad hygienic surroundings appears to be distinctly more 

 vulnerable to many infectious diseases than he is when his environ- 

 ment is more sanitary. Unhygienic conditions, however, are rela- 

 tively complex in their reactions on man, for the attendant evils of 

 overcrowding, underfeeding and increased exposure to infection 

 undoubtedly play a part. 



Heredity also appears to be an important factor in determining the 

 average severity of infection in certain types of endemic disease. 

 Measles is a common and usually fairly mild disease of childhood 

 among civilized people. Among aboriginal populations, as those of 

 the South Sea Islands where the inhabitants had not been exposed 

 to measles previous to the advent of Europeans, the introduction 

 of the virus has resulted in a veritable plague during which large 

 numbers of the people died. This phenomenon of hereditary acquired 

 tolerance for specific endemic disease may conceivably be even more 

 specific; for example, strains of a given organism might produce 

 mild disease in areas where it has been endemic for generations and 

 yet be rapidly fatal for alien populations who may have in turn become 

 partly tolerant for other strains of the same organism. If such prove 

 to be the case, unrestricted emigration may lead to temporary dis- 

 turbances in the balance between specific microorganisms on the one 

 hand and hosts on the other a feature which Theobald Smith called 

 attention to many years ago. 1 



Racial differences in susceptibility are occasionally met with even 

 in the same species. Negroes and Indians are more susceptible to 

 infection with tubercle bacillus than the Caucasian race. The Jews 

 appear to be somewhat more resistant to infection with the tubercle 

 bacillus than the other branches of the Caucasian race. 



H. INFECTION. 



Pathogenic bacteria which reach the host do not necessarily incite 

 disease; they may be, and undoubtedly are, frequently overcome by 

 the body without inducing symptoms. This initial resistance to infec- 

 tion involves an initial struggle between host and microorganism which 

 brings into play non-specific lines of defense of the macroorganism 



1 Theobald Smith, Tr. Assn. Am. Phys., 1893. 



