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presenting fever at the time of their birth, under circumstances which made 

 it highly probable that they had received the Yellow Fever infection "in 

 utero", through their immune mothers. 



From all the above circumstances, it may be inferred that the only 

 protection provided by nature against a severe attack of Yellow Fever 

 consists in the partial or total immunity which results after a previous 

 attack, however mild its outward manifestations may have been. 



If we now turn our attention to the modes of propagation of the infec- 

 tious element we find it most difficult to reconcile them with those of other 

 transmissible diseases. Observation proves indeed that, within limited 

 distances, the infection may be transmitted through the atmosphere ; but the 

 infectious principle cannot be regarded as a passive element diffused 

 through the air, as it is not found to spread in accordance with air 

 currents, nor with the direction of prevailing winds. When the hold of a 

 ship has given decided evidence of a most virulent infection, it seems 

 difficult to identify the pathogenic cause as a passive substance floating in 

 the air or deposited upon the objects in the hold. In such a case the infection 

 has been known to spread over the deck of a ship and in its immediate 

 vicinity, and has also been conveyed to distant places apparently through 

 fomites and human intercourse, while a whole cargo of sugar boxes which 

 had been removed from the same hold and distributed in various directions, 

 never developed a single case of Yellow Fever along its track (as was 

 particularly ascertained in the ease of the "Anne Marie" at St. Nazaire, 

 in 1861). 



Neither is the infection transmitted through water or food, for in esta- 

 blishments where a number of susceptible persons take their meals in common, 

 the order in which they are attacked does not bear out the idea that they 

 had been simultaneously infected. Direct contact with a Yellow Fever 

 patient and prolonged assistance in the sick room do not necessarily deter- 

 mine an attack of the disease; and it never does so in localities where the 

 temperature is low, nor in those which are situated at a considerable al- 

 titude above the sea. Under the two last circumstances the disease is in fact, 

 intransmissible. 



In attempting to reconcile the apparent incongruities which beset the 

 etiology of Yellow Fever, I came to the conclusion some 13 years ago that 

 they might be explained by admitting that the Yellow Fever poison is 

 only pathogenic when introduced by inoculation, and that this operation 

 might be performed by some species of mosquitoes which are peculiar to 

 yellow fever countries. This hypothesis appears to suit indeed all the con- 

 ditions of the problem, as I have shown in various publications, and more 

 particularly in a paper which I contributed for the Congress of Clima- 

 tology, at Chicago, U. S. A., in May, 1893. 



One of the conditions of my mosquito theory, however, might appear 

 somewhat forced, unless confirmed by actual observation. I refer to the 



