The final confirmation of the rôle which appertains to the culex mosquito deso (now included in the genus stegomyia of Theobald) in the transmission of yellow fever, has now been sanctioned by the experiments of Drs. Reed, Carroll, Agramonte and the lamented Dr. Lazear, at Quemados de Marianao, during the winter of 1900, and afterwards by those of Dr. Guiteras at the Experimental Station of Las Animas, last summer, and finally by the splendid practical results obtained by the Chief Sanitary Officer of Havana, Major W. C. Gorgas, during the epidemic year which has just been completed. With those facts and the ones which I had gathered in former years, it is now possible to determine with some degree of precision the conditions which are necessary in order that yellow fever may develop in an epidemic form in a given locality, not too highly situated above the sea level and where temperatures between 25° and 35° C. (77° and 95° F.) either temporarily or habitually prevail. Dr. Reed and I discussed on several occasions the possibility of making practical application of his discoveries. It did not seem to us possible to destroy the adult mosquito in sufficient numbers to be of any practical use. And we were not sufficiently familiar at that time with the life history of the mosquito to think of any other way in which the subject could be approached with a fair prospect of success. We, of the Sanitary Department, after a great deal of discussion and thought given to the matter, decided that we should adopt all measures that seemed likely to be useful, being guided in our plans principally by the life history of the mosquito. After the first two cases—in all some twenty-six—all the Board’s cases had been very light. We concluded, therefore, that our strongest measure would be vaccination; that is, to have an infected mosquito bite a non-immune and give him a light case. If this proved as successful as had vaccination in smallpox, we could see that this measure alone would entirely protect against yellow fever, just as had vaccination against smallpox. We consulted the military governor, General Wood, on the subject, and he agreed to let us try it as one of the several preventive measures which we were putting into effect against yellow fever. We announced that we were ready at Las Animas Hospital to immunize against this disease anyone who wished to be treated in this way. There was no lack of applicants. This was in February, 1901. Most of the work of the Army Board had been done during the preceding fall, and at this time we had only one infected mosquito left, which had been given us by Dr. Reed. This old lady was a veteran in every sense. She had given several people yellow fever, but her greatest claim to celebrity was the fact that it had been fifty-seven days between the first case of fever and the last one which she had given.