YELLOW FEVER 257 



When a case is imported into one of our Southern 

 seaport cities from Havana, Vera Cruz, or some other 

 endemic focus of the disease, an interval of two weeks 

 or more occurs before secondary cases are developed 

 as a result of such importation. In the light of our 

 present knowledge this is readily understood. A 

 certain number of mosquitoes having filled themselves 

 with blood from this first case, after an interval of 

 twelve days or more, bite non-immune individuals 

 living in the vicinity, and these individuals, after a 

 brief period of incubation, fall sick with the disease ; 

 being bitten by other mosquitoes they serve to trans- 

 mit the disease through the " intermediate host " to 

 still others. Thus the epidemic extends, at first 

 slowly from house to house, then more rapidly, as 

 by geometrical progression. The results reported 

 by Dr. Reed and his associates have since been fully 

 confirmed by their subsequent experiments and by 

 independent investigations made in Cuba and also in 

 Brazil. 



Before the discovery that yellow fever is trans- 

 mitted by mosquitoes, this disease was generally re- 

 garded as one of the filth diseases, although there 

 were many facts opposed to this view. In the light 

 of our present knowledge we can no longer class it 

 with typhoid fever, cholera, bubonic plague, and dys- 

 entery, in which diseases the germ is known to be 



