IMMUNITY 181 



transmission of yellow fever by the mosquito, that the natives are 

 wholly immune to this disease. Now, it is quite evident that all have 

 it lightly in infancy, and the adult is protected by an acquired immunity. 

 When syphilis first appeared in Europe, it manifested a malignancy 

 much greater than that now usually observed in the white man. It 

 is said to be still much less malignant among the Indians of Central 

 America, whose ancestors, far back beyond the Columbian period, 

 probably had the infection. On the other hand, people among whom it 

 has recently been introduced, suffer its full virulence. The increased 

 mortality of smallpox, measles, scarlet fever and tuberculosis, among 

 peoples unaccustomed to these diseases, seems to be well authenticated. 

 We must conclude that increased resistance to specific infection is both 

 inherited and acquired. 



Variation in individuals of the same species to a given infection is 

 a most interesting subject and one which cannot at present be wholly 

 explained. As a rule, the more virulent the infection the less marked 

 the variation. When many guinea-pigs are inoculated with a fairly 

 virulent anthrax culture in the same dosage, all die about the same 

 time. When rabbits are treated in the same way with anthrax, some 

 die within one or two days, others live longer and some do not die. 

 When rabbits are inoculated with the bacilli of chicken cholera, there 

 seems to be no resistance. The reaction at the point of inoculation is 

 slight, a general septicemia results and all the animals die. When 

 guinea-pigs are inoculated with the same organism there is a marked 

 local reaction, as a rule there is no general infection and but few die. 

 When guinea-pigs are inoculated with a virulent culture of the human 

 tuberculosis bacillus all die. With rabbits only a small proportion die. 

 The water-supply of a city becomes infected with cholera or typhoid 

 bacilli. From 5 to 20 per cent, of those who drink the water may 

 develop the disease. Why does not this happen to all? Some answer 

 by saying that the bacilli, being particulate, are not uniformly dis- 

 tributed through the water, consequently all who drink the water do 

 not swallow the infective agent. All soldiers who make a charge in 

 the face of continuous musketry discharges are not killed or wounded. 



There is much of truth in this explanation, but it is not altogether 

 applicable. Many of those who do swallow the bacilli, as can be dem- 

 onstrated by their presence in the stools, do not develop the disease. 



