50 IMMUNOLOGY 



2. Racial Freedom from Disease May Be Due to Customs 

 Rather Than Immunity 



Effect of Customs. — The Hebrew people do not suffer from 

 trichinosis. Since trichina infection is obtained by eating infected 

 or "measly" pork it is obvious that this is not an example of im- 

 munity but that freedom is due to racial taboo of pork. Since 

 cases have been reported among Hebrews who violated the taboo 

 accidentally or intentionally, we have no reason to suspect im- 

 munity. 



3. Environmental Conditions: Absence or Presence of Factors 

 Affecting- Immunity Mechanisms 



Climate Factors. — Dysentery is much less prevalent in the tem- 

 perate and northern climates than in the tropics. It has been sug- 

 gested that this illustrates the devitalizing effect of heat. It 

 should not be assumed, however, that heat is the only factor. 



Lobar pneumonia and upper respiratory infections are said to 

 be much less prevalent in the tropics than in the colder climates. 

 The inference is made that this may be due to the effect of tempera- 

 ture, humidity, and air currents on the mucous membranes, lower- 

 ing their resistance. The so-called "fog pneumonia" in England 

 and Belgium reported in 1930 is probably an example of this type. 

 Many occupational diseases come under this group. 



4. Resistance and Susceptibility Factors 



It seems quite evident from the experimental studies of Topley, 

 Wilson, and Greenwood in England and Webster and others in 

 America that in so far as mouse typhoid is concerned, the addition 

 of fresh mice to an infected popuhition initiates an epidemic. 



Webster and Ilodos have shown that the mortality, during an 

 epidemic, is conditioned by the number of highly susceptible con- 

 stituents. When the latter are few in number the deaths will be 

 sporadic; if the number of highly susceptibles is great the mor- 

 tality will be of epidemic proportions; if the susceptibles are de- 

 pleted the mortality subsides. 



When recruits are added to sucli surviving populations, made 

 up chiefly of resistants, Webster and Ilodes find that mouse typhoid 

 infection spreads to both resistant and susceptible recruits but that 

 the mortality is limited to the susceptible recruits. The innately 



