249 



of cases, the danger varies greatly. The majority, in some groups, may be 

 cases which would be cured under any form of treatment, while in others 

 there may be en unusual number of cases of the worst description which 

 seem from the first doomed to die in spite of all efforts and under any 

 treatment that can be devised. 



Mode of Transmission. — This is a subject which has tried the sagacity, 

 of all the observers who have attempted to unravel the etiology of yellow 

 fever. Even now the statement is occasionally met with, that the disease 

 may be contracted by ingesting contaminated food or drink, or by inhaling 

 disease germs that may be floating in the air. Yet a close scrutiny of the 

 order in which the cases occur, and of the peculiarities that are observed in 

 the propagation of the disease, gives no support whatsoever to those asser- 

 tions. Some of the objections have already been pointed out (see "Nature 

 of the Disease"), but there is another important one that will now be 

 mentioned. It is a well-established fact that beyond certain limits of 

 height above the level of the sea the disease cannot be acquired either 

 through fomites imported from infected localities, or by direct contact and 

 frequentation of yellow fever patients who may happen to be in those 

 high levels (having received the infection elsewhere). Low temperatures 

 produce, the same results as high levels, though each, we are told, 

 independently of the others. Now, to my mind, this would be quite incom- 

 prehensible if the disease could possibly be acquired either by inhalation, 

 ingestion, or contact. Some thirteen years ago I was led by these considera- 

 tions to think that the only mode of transmission at all compatible with the 

 ascertained facts must be by "inoculation." A simple reasoning further 

 induced me to suspect that such inoculations of the disease germs might 

 be produced by some stinging insect peculiar to yellow fever countries, and 

 which, for some unknown reason, should alone possess the requisite condi- 

 tions for retaining those germs in their full potency and vitality. Some 

 species of gnat might well fulfill some of the requisites ; for although the 

 genus culex is distributed pretty much over the whole world, the different 

 species have distinct climatic requirements. A close investigation into 

 the habits and the biological conditions of the species of gnat commonly met 

 with in Havana and in other places along the Atlantic coast of tropical 

 America soon convinced me that those which come out during the day 

 time, at dawn, and in the evening, — the ' ' culex mosquito ' ' in particular, — 

 unite the most essential conditions to fulfill the role of a special inoculating 

 agent which should be the habitual propagator of yellow fever. How closely 

 the vital conditions of this mosquito agree with those which are known to 

 favour or hinder the propagation of that disease may be seen in the following 

 paradigm: 



