146 MALARIA IN RELATION TO MAN [CH. 



has taken in but a few parasites. Thus questions of suscepti- 

 biUty and immunity, whether natural or acquired, or due to 

 the use of drugs, have to be considered. Also the condition 

 of nutrition of the members of a community, and its effect 

 on the natural resistance of the human organism against the 

 parasite, and the extent to which influences favourable to 

 relapses prevail, are all factors in the epidemiology of malaria. 



Susceptibility to malaria may be evidenced either by in- 

 creased liability to infection, or by the degree to which toleration 

 of the presence of the parasite exists. In the case of primitive 

 and aboriginal races there is evidence to shew that apart from 

 any possible lessened liability to infection, there is often a greater 

 degree of toleration than is exhibited, for example, in Europeans. 

 A real immunity with a lessened liability to infection is also 

 undoubtedly developed in those persons long exposed to 

 malarial infection. When in any race toleration and immunity 

 are well marked, a considerable degree of malaria may occur 

 in a latent form. In such cases susceptible strangers suffer 

 far more than the native residents. 



The importance of the part played by susceptibility and 

 immunity in malaria was demonstrated by Koch at Stephansort 

 in New Guinea, where, at the time his observations were made, 

 malaria was very prevalent. Koch was able to shew that after 

 a batch of Chinese immigrant coolies, who formed a large portion 

 of the population, had been resident for some years in the 

 colony they suffered distinctly less from malaria than they did 

 on their first arrival, and that in the absence of fresh immigration, 

 malaria amongst the community, as a whole, shewed a steady 

 decline, but with each successive large immigration of new 

 coohes there was a marked recrudescence of the disease. These 

 observations of Koch establish a principle in malarial epide- 

 miology which may be termed the law of non-immune immi- 

 gration (Christophers and Bentley), and which applies to all 

 immigration of susceptible individuals into a malarial focus. 



Where the number of strangers is small, the effect is chiefly 

 exhibited upon them, as in the case of Europeans residing in 

 Africa. Here, as shewn by Stephens and Christophers, the 

 whole epidemiological outlook as regards the European is 



