IX] MALARIA IN RELATION TO MAN 147 



dominated by the presence of a more or less latent native 

 malaria. 



Where the influx of strangers is comparatively very large, 

 the whole community is adversely affected, and under certain 

 circumstances the prevalence of malaria through a whole tract 

 of country may be greatly increased. 



Since the new-born child is, in a sense, an immigrant, in 

 any community the young children are especially affected. 

 Where malarial infection is intense, but the population is stable 

 and possibly possesses a certain degree of racial tolerance, the 

 amount of infection is highest in the young children and at a 

 minimum in the adults. In this case the children go through 

 life more or less permanently infected and with a certain amount 

 of splenic enlargement. At about the age of puberty, they 

 cease to shew parasites in their blood, or to exhibit enlarge- 

 ment of the spleen. 



Where malaria is more or less seasonal in its occurrence, 

 or where there is any serious degree of immigration or shifting 

 of populations, the restriction of infection to children is less 

 marked. All grades of difference in this respect may be en- 

 countered, the usual rule being that strictly indigenous and 

 primitive tribes shew the greatest amount of tolerance, and 

 along with a high proportion of infection amongst children, 

 exhibit the greatest adult immunity from infection. 



Recently much stress has been laid upon the influence of 

 economic factors in determining a high or a low degree of 

 malaria prevalence. Poverty stricken and squalidly living 

 communities in malarious countries usually exhibit a far 

 greater degree of malarial infection than do communities in a 

 condition of prosperity. Besides the " social " distribution of 

 malaria, very grave manifestations may be brought about at 

 least partly by conditions of general stress, such as the occur- 

 rence of famines or scarcity following failure of rainfall, especially 

 when this is followed by floods still further adding to the distress 

 and supplying all the requisites for an active transmission of 

 the disease by Anopheline mosquitoes. Thus, in Denmark, 

 epidemics of great severity have frequently followed great 

 storms, which had led to extensive inundations associated 



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