That after the non-immune human being had been bitten by the infected stegomyia mosquito, an incubation period of from three to six days elapsed before the man began to show symptoms of yellow fever. The shortest period of incubation in Dr. Reed’s cases was two hours less than three days, and the longest period, two hours more than six days. That the blood, taken at the proper time and injected into a non-immune, would also cause yellow fever; that the disease was caused by a parasite, and that the parasite was sub-microscopic. These discoveries have been of enormous benefit to mankind, and upon them has been based the sanitary work against yellow fever which has been so successful. At first blush, however, it was not evident to what extent it would be of practical use to us. CHAPTER IV THE SANITARY BOARD OF HAVANA At this time, February, 1901, I was health officer of the city of Havana. The efforts of the Department had been concentrated for more than two years previous upon controlling yellow fever in that city. Not only had we met with no success, but yellow fever was actually worse than when we commenced work. Ample funds and power had been given us by General Wood, the military governor, and we had by far the best and most efficient sanitary organization of whose existence, either before or since that time, I have any knowledge. When the work of the Reed Board began to point to the mosquito as the conveyor of yellow fever, the Sanitary Department of Havana was at its wits’ end and was glad to receive this discovery as a means of possible help. I, as health officer of Havana, had nothing to do with the work of the Reed Board in any way whatever, except that I was a very interested spectator and kept in close touch with the work as it developed. We assisted the Board in every way that we could. All the hospitals in the city were under our control, so that we were enabled to furnish them ample clinical material. Neither the Reed Board nor any of its members had anything to do with the practical working out of the methods whereby their theory was demonstrated, and by means of which yellow fever was finally eliminated from Havana. These methods were first originated and worked out by the Sanitary Department of Havana during the year 1901. They have since been copied and successfully applied in many parts of the world where yellow fever formerly prevailed. Dr. Finlay, in a reprint from the _Journal of the American Medical Association_, April 19, 1902, says: