NATURAL AND ACQUIRED IMMUNITY 107 



mouse population but tlieir resistance measured in terms of the 

 killing potency of the organism was altered. They conclude, how- 

 ever, that in exposed or infected populations of mice the mortality 

 is conditioned, primarily, by the number of highly susceptible 

 constituents. 



These discussions of hereditary and nutritional factors are in- 

 cluded in this chapter since they play a role in determining both 

 the morbidity and mortality in large groups as well as in individual 

 cases. 



Neufeld's Theory of Phases of Heightened and Lessened 

 Resistance. — Neufeld and his colleagues, Etinger-Tulczynska 

 (1933) and Kuhn (1933), hold to somewhat different views. They 

 explain the apparent immunity observed among survivors of 

 experimentally infected mice as due to the testing of the animals 

 during a highly resistant period. They seemed to think that there 

 are periods of increased and decreased natural resistance. During 

 an epidemic some individuals will be in the phase of heightened 

 while others will be in a phase of lowered resistance. The former 

 constitute the group of survivors whose immunity is ordinarily at- 

 tributed to previous infection but which Neufeld regards as due to 

 the fortuitous circumstance that they are in the phase of heightened 

 natural resistance, whatever that may mean. Undoubtedly many 

 factors operate. Hirszfeld (1927) thinks that he has demonstrated 

 hereditary factors in natural immunity to diphtheria. His con- 

 clusions have been severely criticized bj^ Snyder (1927), Rosling 

 (1928) and others. 



Hereditary Factors in Natural Immunity. — ^Webster and his 

 colleagues, Burn and Pritehett, conclude from their studies of 

 microbic virulence and host susceptibility that hereditary factors, 

 microbic distribution, microbic virulence and dietary factors are 

 all quite important in determining whether infection occurs, the 

 severity, the development of the carrier state and also the dura- 

 tion and complications of the disease in mice and rabbits. Lambert 

 (1932) reported that by selective breeding he could produce in- 

 dividuals more resistant to the organisms of fowl typhoid than 

 those in the unselected stock. 



Since the latter paper appeared, Irwin (1929, 1933) has pub- 

 lished the results of four years' studv of the role of inheritance 



