100 Disease and Inununily 



immune before would have lost their powers of 

 resistance by reason of the fact that there was 

 no occasion to exercise them. The disease would 

 again sweep away thousands and again dis- 

 appear for years. But when vaccination came 

 into vogue there was no long idle period for an 

 entire population, or even for a small part of it. 

 The powers of resistance became inherited as far 

 as such powers existed at the time of reproduc-? 

 ing. 



The difference between measles and smallpox 

 in this matter is that measles did not come by 

 epidemics separated by many years in which 

 there were no cases in a community of consider- 

 able size. Usually measles has been continually 

 present, with the result that some part of the 

 population was continually becoming immune. 

 The child caught the disease and was thereafter 

 immune. By the time the next generation came 

 along, the immunity had declined sufficiently to 

 leave the child subject to a new attack, but not 

 enough to make the disease dangerous. The 

 mere fact that there is a germ between parent 

 and child does not affect the matter in the least. 

 In those powers which are built up by exercise 



