96 Disease and Immunity 



of terrible scourges, the actual deaths are only a 

 small fraction of the entire population, a fact 

 which shows that the average resistance is greater 

 that the average powers of attack by whatever 

 parasite it is that causes the disease. It is well 

 known that some races of men have greater re- 

 sistance to certain diseases than have other races, 

 and that some members of any group have 

 greater resistance to certain diseases than have 

 other members. It is also known that the same 

 person has greater resistance at some times than 

 at others, as is evidenced by the fact that he 

 will contract a disease at one time and not at 

 another, even though exposed. 



When the resistance to a particular disease is 

 of a high order, it is called immunity. Thus, 

 the negroes of the West Indies are practically 

 immune to yellow fever and malaria, the Chinese 

 are said to be immune to cholera, the buffalo and 

 Texas cattle are relatively immune to the disease 

 caused by the Texas fever tick, the native Ameri- 

 can grape-vines are highly resistant to the 

 phylloxera, which has played such havoc with 

 European vineyards, and Asiatic chestnuts are 



