THE INHERITANCE OF DISEASE 495 



with the disease are apparently influential factors about 

 which there is as yet insuflicient information. 



When we turn from the race to the individual, vision 

 apparently becomes clearer, for there can be no doubt 

 that with reference to most infectious diseases there are 

 wide individual variations in resistance. These are made 

 manifest in several ways. Most certainly perhaps in the 

 varying severity of the effects of an established infection, 

 but also in all reasonable probability in the "take or no take" 

 as a result of approximately equal grades of exposure. It 

 is again remarked that the matter is apparently clearer 

 when the individual is considered. It is meant that the 

 differences in resistance are more definitely discernible, 

 they are in fact unmistakable. But in the individual case 

 it is always open to question whether the exposure has 

 in fact been equal; whether more or less immunity has been 

 acquired from the mother, or actively accumulated through a 

 succession of abortive exposures; or whether a previous 

 mild attack of the disease may not have passed unnoticed 

 and given an effective vaccinal protection. The great 

 advance in medical science in the past fifty years consists 

 in considerable part in the acquisition of the understanding of 

 these fundamental features of the body's reaction to infections. 

 It has seemed to many, perhaps to most, thoughtful physicians, 

 in recent years that these near at hand factors were sufficient 

 to account for all the differences in individual resistance. 



But if we go back for a moment to an earlier period we 

 find a fixed and universal opinion that certain infectious 

 diseases follow family lines to a considerable extent. This 

 is not true of measles or smallpox. It seems conspicuously 

 true of tuberculosis. Most of us can doubtless call to mind 

 families in which severe illnesses and deaths from tuberculosis 

 have been common, and other families in which they have 

 been rare. Large groups of family histories have been 

 collected and. submitted to the best available mathematical 

 analysis and these have also given evidence of some difference 

 in the inheritance. But it is also known that under conditions 

 of universal exposure as in crowded cities, practically all 

 individuals have some tuberculosis at some time or other. 

 The disease is one which often lasts in individual cases for 



