Wherever yellow fever is endemic, it is a well-recognized fact that the native of the endemic area is not subject to this disease. This is explained on the theory that he had a mild attack in childhood, which, though not recognized, gives him protection in after life. This statement may strike one with surprise at first. We have, however, an exactly similar state of affairs among cattle. The beef native to a Texas fever region does not suffer from the disease, but an animal brought from anywhere outside this region always contracts the disease, and generally dies. The calf of the native cow is believed to have a mild attack which does not make it seriously sick, but protects it from Texas fever all through life. If the calf of the foreign cow is born in the endemic area, it seems to survive just as does the native calf, though the mother may have died of Texas fever. An adult human being frequently has yellow fever in so mild a form that it is not recognized as yellow fever. Many Europeans are found in a yellow-fever endemic center such as Havana, who have lived there for years and not had yellow fever, as far as they themselves were aware. If, therefore, an individual could prove that he had lived for ten years continuously in a yellow-fever center, his immunity was accepted by the quarantine authorities. If a ship had touched at a port where yellow fever prevailed, before coming to Panama, there was a possibility of infected stegomyia having gotten aboard, even though no cases of yellow fever had developed on the ship. For by chance the infected mosquitoes may not have bitten anyone, or if they had bitten persons on the ship, the biting may not have taken place a sufficient length of time for the development of the disease, before the arrival of the ship at Panama. Such a ship was considered as possibly infected, and was treated at quarantine exactly in the same way as above described in case of a ship known to be infected. The fumigation of a ship was generally accomplished by burning sulphur, as above described in the case of fumigating a house. In the parts of a ship, such as the engine-room, where there was valuable machinery which would be injured by sulphur fumes, pyrethrum was used, just as in similar circumstances it was used in fumigating dwelling-houses. We had more elaborate machinery than the pots and pans described in the fumigation of houses, with which sulphur fumes could be developed much more rapidly and in much larger volume. This was used by us on special occasions where its use seemed desirable.