XI] 



HISTORY 



179 



greatest difficulties in the way of studying the disease is the 

 fact that mild cases of yellow fever cannot be diagnosed 

 with any certainty. As a result, although the natives of 

 endemic regions must frequently harbour the virus in their 

 blood, up to the present it has been impossible to discover 

 any direct proofs of it. 



140 



120 



100 



20 



20 



140 



120 



Fig. 50. The approximate distribution of Yellow Fever at the present time. 

 Infected regions are coloured black. 







The cause and method of infection of yellow fever have 

 been a puzzle ever since the disease became known, and although 

 the latter of these two points has now been settled the causal 

 agent is still a matter of discussion. 



In early times it was believed that the virus was in some 

 way transmitted through the air, and the fact that it usually 

 occurred in the vicinity of water led to the belief that it was 

 produced by bacterial fermentation. Subsequently a number 



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