RACES IMMUNE TO YELLOW FEVER 171 



three days, the virus incubates in its body for about 

 twelve days, and at the end of this time the mosquito 

 is most heavily infective, and its bite produces acute 

 disease. Needless to say, strict regard must be given 

 to cleanliness and the maintenance of quiet in and 

 about the sick-room. Food should not be allowed 

 until the temperature has been normal for a day or 

 two, and when the kidney action has been re- 

 established. Complications are uncommon, and the 

 patient usually recovers completely, and has acquired 

 a more or less persistent immunity. 



In many tropical places it has been stated that 

 there is a periodicity in the appearance of yellow 

 fever. In some parts of Africa, the natives believe 

 in a seven-yearly outbreak, while in Barbadoes the 

 period is cited as thirteen years. Investigation shows 

 that there is no such periodicity. Outbreaks occur 

 in the tropics after the rainy season, since with much 

 water in all sorts of receptacles the Stegomyia have 

 endless chances of breeding. Cold makes mosquitoes 

 torpid and retards egg-laying; consequently, "frost 

 puts an end to an epidemic." 



Certain races have been stated to be immune to 

 yellow fever. Negroes undergo slight, scarcely 

 recognizable attacks when young, and acquire im- 

 munity. In an epidemic, the non-immune whites 

 succumb, while as a rule the blacks are not much 

 affected. These immune races are, then, in the 

 position of virus carriers. Themselves infected, they 

 feel no ill-results, but the Stegomyia can obtain the 

 virus from them and transfer it to non-immune 

 people. Europeans born in yellow fever centres 



