The immunity which the West Indian negro enjoys from this disease gave him a superiority over other labourers on the Isthmus which, since the extinction of the disease, is no longer his.
During the American occupation of Havana, after the American-Spanish War, yellow fever broke out among the strangers, and the mere cleaning up of the city, though carried out with military thoroughness, had no effect in checking the disease. A medical board was sent to study the matter. This was in 1900, four years after Major Ronald Ross, of the Indian Medical Service, had discovered the cause of malaria. Ross had proved that the cause of malaria in man was the presence in his blood of an organism introduced by the attack of the anopheles gnat (or mosquito), and that the species was only poisonous to man if it had itself become infected with the germ of this organism in biting a man suffering from malaria. Thus man and anopheles act alternately as hosts to the organism, which apparently requires their co-operation for the continuance of its species.
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.
READING ROOM, EMPLOYEES' CLUB, CULEBRA.