366 



that, after the lapse of a proper interval of days, a bite from the same 

 insect will almost certainly develop a mild attack in the non-immunes 

 whom the insect may thereafter sting, and the contaminated mosquito will 

 retain that faculty during the remainder of its life. 



The value of this discovery is, to my mind inestimable, not only for 

 having enabled the experiments to produce absolutely convincing proofs of 

 the transmission, but for opening the way toward a very great improvement 

 in my former methods of preventive inoculations, so that we may confidently 

 look forward to a process by which immunity will be safely conferred 

 within a brief space of time to new-comers who may be willing to undergo 

 the comparatively slight inconvenience of a mild experimental attack of 

 the disease. 1) 



Although this important discovery was made in accordance with 

 precedents derived from certain known facts concerning the malaria 

 infection, it does not follow that the germ of yellow fever must, of necessity, 

 also be an animal parasite. The circumstance that, by allowing a certain 

 interval of time between the contamination of the insect and the ino- 

 culation, the efficacy of the latter is undoubtedly enhanced, I had myself 

 surmised, without taking into account that the germ might require to 

 go through any special transformations within the body of the insect ; 

 my idea was simply that the prolonged contamination would allow 

 the germ to multiply to such an extent that a more abundant supply would 

 be secured for the salivary and venom glands of the infected mosquito. 

 Regarding the curious fact that the contaminated insect retains the power 

 of reproducing the disease during its life, this is not a privilege ap- 

 pertaining exclusively to animal parasites, since it is also observed with 

 regard to the bacilli of leprosy and of tuberculosis in man. 



As a practical demonstration that fomites, in the usual sense of the 

 term, are incapable, per se, of conveying the yellow fever infection, the 

 experiments at Camp Lazear were very significant and most ingeniously 

 devised. The generalization of the principle is, however, only justified by 

 the circumstance that the outcome of those experiments affords a direct 

 corroboration of what actually occurs in nature, experience having 

 repeatedly shown that in localities where the disease appears to be 

 intransmissible — as in the City of Mexico, in Petropolis, and, in 1853, at 



1) The innocuousness of the method would still be grounded upon the same 

 argument which I submitted to Captain-General Blanco, in 1881, when I solicited his 

 permission for my first experiments with contaminated mosquitoes, and which at once 

 appealed to his clear-sighted judgment. "If," I argued, "the mosquito is truly the 

 indispensable agent of transmission which I conceive it to be, the mildest, recognizable 

 attacks of yellow fever that occur in nature must be attributed to the smallest dose of 

 the yellow fever virus that may be introduced into the non-immune, a condition which 

 ought surely to be fulfilled by applying only one mosquito which has only once before 

 stung a yellow-fever patient.' ' The great improvement which results by reason of the 

 recent discovery consists in the facilities which it affords for obtaining direct evidence 

 that the inoculation has been successful. 



