YELLOW FEVER 123 



met with. The early cases in an epidemic are twelve to 

 fifteen days after a case is introduced, as the mosquitoes 

 have to be infected and become infective, which takes 

 ten days, and then the person infected by them will not 

 develop the disease for two, three or four days. The 

 incubation period for an epidemic is therefore about two 

 weeks. Most of the etiological factors connected with 

 yellow fever are closely related to the conditions favour- 

 able for the life and multiplication of S. fasciata, but this 

 insect is more widely spread than yellow fever, and is 

 only of importance, in this connection, when persons 

 with yellow fever are present or are imported, or where 

 the disease is endemic in a mild or severe -form. 



Prophylaxis. The carrier of yellow fever is known, 

 and for effective prophylaxis thorough knowledge of 

 the habits of this insect is required, and the means of 

 identification. It is possible that other members of the 

 same group may carry yellow fever. 



The point of special importance is the class of persons 

 who harbour the parasites and serve as a reservoir of the 

 disease from which these mosquitoes become infected. 

 The general recognition of the presence during an epi- 

 demic period of numerous cases of illness, characterized 

 by brief fever and gastric disturbance, with occasionally 

 jaundice, some at least of which are probably yellow fever, 

 has broadened the scope of inquiry, 



Such cases, as well as the more marked cases of yellow 

 fever, serve as " reservoirs " of the disease and mosquitoes 

 become infected from them and in turn infect other 

 persons. 



As the mild cases are the usual forms of the disease in 

 children, and the immunity conferred by a previous attack, 

 whether mild or severe, is rather immunity against a 

 severe attack than against reinfection with the disease, 

 it is held by some that in tropical countries, where the 

 mosquito S. fasciata is common, such constant reinfec- 

 tion occurs amongst the native children and the partially 

 immune adolescents and adults that the disease persists 



