Arriving at the army barracks near Havana the Commission first took up the study of _Bacillus icteroides_, the organism which Sanarelli, an Italian physician, had declared the causative agent in yellow fever. They were unable to isolate this bacillus either from the blood during life or from the blood and organs of cadavers and therefore turned their attention to Finlay's theory of the propagation of yellow fever by means of the mosquito. In this work they had the unselfish and enthusiastic support of Dr. Finlay himself, who not only consulted with them and placed his publications at their disposal, but furnished eggs from which their experimental mosquitoes were obtained. Inoculations of eleven non-immunes through the bite of infected mosquitoes were made, and of these, two gave positive results. The first of the two was Dr. Carroll who allowed himself to be bitten by a mosquito which had been caused to feed upon four cases of yellow fever, two of them severe and two mild. The first patient had been bitten twelve days before. Three days after being bitten, Dr. Carroll came down with a typical case of yellow fever. So severe was the attack that for three days his life hung in the balance. During his convalescence an incident occurred which showed how the theory of mosquito transmission of the disease was generally regarded. We quote from Dr. Kelly: "One of his nurses who came from Tennessee had had considerable experience with yellow fever, having indeed, lost her husband and several children from it. One day early in his illness Dr. Carroll mentioned to her that he had contracted the disease through the bite of a mosquito, and noticed that she looked surprised. Some time later, when well enough to look over the daily records of his condition, he found this entry: 'Says he got his illness through the bite of a mosquito,--delirious'." The second case was that of an American who was bitten by four mosquitoes, two of which had bitten severe (fatal) cases of yellow fever twelve days previously, one of which had bitten a severe case (second day) sixteen days before and one which had bitten a severe case eight days before. Five days later, the subject developed a well pronounced but mild case of the disease. In the meantime, another member of the Commission, Dr. Lazear, was accidentally bitten by a mosquito while collecting blood from yellow fever patients. Five days later he contracted a typical case which resulted fatally. So clear was the evidence from these preliminary experiments that the commission felt warranted in announcing, October 27, 1900, that, "The mosquito serves as the intermediate host for the parasite of yellow fever, and it is highly probable that the disease is only propagated through the bite of this insect."