Gnats, or mosquitoes, as they are indifferently termed, being thus under more than suspicion as an immediate cause of tropical fevers, the medical board turned their attention to them, and Mr. Reed, a member of the board, tracked the yellow fever to another gnat, the _stegomyia_, and, aided by the heroic devotion of his assistants, proved beyond shadow of doubt that this disease is due to the activity of another minute organism, which lives a double life in man and _stegomyia_. Mere contact with the clothing, &c., of yellow-fever patients was proved to be no source of infection. The _stegomyia_ lives three months. It becomes dangerous only by imbibing the organism through attacking man during the first three days of yellow fever, and, even then, twelve days elapse before its bite is infectious. Six days after a man has been bitten by an infectious _stegomyia_ he develops yellow fever, and for the next three days (as has been already said) he is infectious to the _stegomyia_. During the American occupation of Cuba attempts were made to obtain immunity from yellow fever, but it was found impossible to regulate the disease when voluntarily communicated by the bite of the mosquito, and at present immunity is only enjoyed by persons who inherit the privilege. The _stegomyia_ does not breed in open swamps or large bodies of water, but needs shelter, and is also incapable of sustaining a long flight. It breeds chiefly in and near towns, depositing its larvæ upon the surface of cisterns or stagnant pools. Colonel W.C. Gorgas, M.D., took charge of the Department of Sanitation of the Commission in July, 1904. "The experience of our predecessors," he writes,[27] "was ample to convince us that unless we could protect our force against yellow fever and malaria we would be unable to accomplish the work." [27] "Sanitation in the Canal Zone," by W.C. Gorgas _Journ. Am. Med. Assoc._, July 6, 1907, vol. xlix. [Illustration: READING ROOM, EMPLOYEES' CLUB, CULEBRA.] [Illustration: HALL OF EMPLOYEES' CLUB, CULEBRA.] At this time there was but little yellow fever on the Isthmus, and, in spite of the arrival of a large number of non-immunes, no alarming outbreak occurred during the first ten months. During April, 1905, however, the administration building in Panama, in which worked some 300 non-immune employees of the Commission, became infected. In that month there were 9 cases and 2 deaths; in May, 33 cases and 8 deaths, of which 21 cases and 2 deaths were among employees of the Commission. In June there were 19 deaths from yellow fever on the Isthmus, and in July 13. The Commission reported[28] that:--