TUBERCULOSIS 127 



living are low or overcrowding or under-nourishment 

 exists, the death rates are higher, and conversely among 

 the wealthier class the amount of disease and death is 

 low. 



IMMUNITY FROM TUBERCULOSIS. Practically no one 

 is immune to infection, but many are immune to the 

 disease. The exact reason for this immunity is not 

 known. Immunity is racial, family, or individual. 

 The Jews and the Italians are relatively immune while 

 the Irish and the Negroes have very little immunity. 

 Family immunity exists, for in many families no evidence 

 of tuberculosis may exist for generations. Then some- 

 times one individual in a family may remain well when 

 all other members become diseased even though ap- 

 parently exposed to the same dangers. Individual 

 immunity varies also, for the immunity is lowered by 

 acute illness, pregnancy, childbirth, long hours of 

 laborious work, and constant undernourishment. 



DANGERS OF INFECTION AND DISEASE. The dangers 

 are twofold. First, the risk of receiving at one time 

 large quantities of tubercle bacilli from a case of ad- 

 vanced pulmonary tuberculosis or from the milk of a 

 cow with tuberculosis of the udder. Second, having 

 one's resistance lowered as a result of ill health or a poor 

 standard of living. In many instances both factors 

 play a part, as when a healthy woman cares for her 

 tuberculous husband, the family income is lowered, the 

 woman's work becomes more arduous and she is con- 

 stantly exposed to infection. There is no considerable 

 danger, however, from the brief exposure of healthy 

 individuals to those affected with tuberculosis which 



