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different from those normally found in countries and seasons where and 

 when the disease does not appear to be transmissible. I have thought, 

 however, that the salivary secretion of the culex mosquito may possess 

 special conditions enabling the yellow fever germs retained upon the 

 sting to preserve their vitality, perhaps to sporulate, during the interval 

 of days which must elapse between two successive bites. There must be a 

 limit, however, to this anomalous vitality of the germ, which probably 

 may not be prolonged beyond four or five days. This would account for the 

 circumstance that I had never obtained any pathogenic reaction, nor any 

 evidence of subsequent immunity, with my inoculations during the winter 

 season, when the mosquito requires a longer time to digest its feed of blood 

 and often will not bite a second time before a lapse of four, five or six days. 

 My technique was a very simple one. The mosquitoes were caught in my 

 own dwelling, at a time when no cases of yellow fever were supposed to 

 exist in the neighborhood, and after having ascertained by their bright 

 markings and general appearance that they were young insects, probably 

 not more than a few days old, each was confined in a small phial or test-tube 

 without food or water. The insects were then taken to one of our hospitals 

 and the yellow fever cases carefully examined until one with severe 

 symptoms could be found, not farther advanced than, the sixth day (usually 

 on the third, fourth, fifth or sixth) of the attack. The mosquito was then 

 made to fill itself with the blood of the patient, and left to digest its feed 

 of blood, imprisioned within the same phial. In the ordinary course of 

 events, in summer, the insect would be ready for another bite after the 

 lapse of 48 to 60 hours ; on some occasions, it would then be made to bite a 

 second yellow fever patient, in which case another interval, of days would 

 have to be allowed before applying the contaminated mosquito to a non- 

 immune. 



I have carried out the above procedure, in collaboration with Dr. 

 Claudio Delgado, upon 102 non-immunes in the course of 17 years (1881 

 to 1898). In 17 of the inoculated persons some febrile reaction occurred 

 after a number of days varying between five and seventeen, in 14 of them, 

 and as late as the 22d, 23d and 25th in the 3 others. The reaction consisted 

 in a mild albuminuric attack of yellow fever in 2 ; in a distinct non- 

 albuminuric attack of the disease in 6 ; in an abortive attack, with or 

 without traces of albuminuria, in 3 ; and only an ephemeral fever, of 

 doubtful diagnosis, in 6. Of these 17, two of the 6 who had experienced 

 only ephemeral reactions showed no immunity at all, having been attacked 

 with severe yellow fever in the course of the same year or the next ; of the 

 3 with abortive reactions, one had another attack of the same character 

 two years later; of the 6 who had developed a non-albuminuric yellow 

 fever after their inoculation, one went through another attack of the same 

 type a few months later, on severe exposure, a fatal case having occurred 

 in the room next to the one which he occupied ; neither of the two who had 



