For many years the cause and methods of dissemination of this disease had been a puzzle to physicians and scientists. Very early it was believed that it might be transmitted through the air, and the fact that infection usually occurred in the vicinity of the water and in the tropics or in midsummer led to the belief that the disease was due to fermentation. This theory received strong support in the fact that serious outbreaks of the fever often followed the coming into port of vessels from the tropics with the water in their holds in an offensive condition. When it was discovered that bacteria were the cause of fermentation and also of many diseases this theory was considered abundantly proven. From time to time, announcements have been made that the particular species of bacteria that causes the disease has been isolated, but there has always been something lacking in the final proof. Yellow fever has always been regarded as a very highly contagious as well as infectious disease, and the utmost precaution has been taken to isolate the patients when possible and in recent years strict quarantines have been established against infected localities and no person or commerce or even the mails were allowed to come from such places without thorough fumigations. But all these things proved unsatisfactory. The disease could not ordinarily be checked by simply isolating the patients. Many people became sick without ever having been near a yellow fever patient, while others worked in direct daily contact with the disease and did not suffer from it. Those who had once had it and recovered became practically immune, rarely suffering from a second attack. Negroes may suffer from the disease, but are usually regarded as practically immune. [Illustration: FIG. 104--Yellow-fever mosquito (_Stegomyia calopus_). (R. Newstead, del.)] It was early observed, too, that the danger zone might be quite well defined and that outside this zone one would be safe. More than a century ago the British troops and other inhabitants of Jamaica found that by retreating to the mountains during the warm weather the non-immunes could escape the fever. It was also observed that those who slept on the first floor were more apt to take the disease than those on the second floor. THE YELLOW FEVER COMMISSION In 1900, during the American occupation of Cuba, yellow fever became very prevalent there. A board of medical officers was ordered to meet in Havana for the purpose of studying the disease under the favorable opportunities thus afforded. This board, which came to be known as the Yellow Fever Commission, was composed of Drs. Walter Reed, James Carroll, Jessie W. Lazear and Aristides Agramonte of the United States Army. Agramonte was a Cuban and an immune, the others were non-immunes. Dr. Manson in his lectures on Tropical Medicines says of them: