There has been some inclination to question the immunity conferred by an attack of yellow fever but Carter has shown that in quarantine practice we can admit such immunes with perfect safety. Thirty thousand such immunes were allowed to enter Key West and Tampa from Havana between 1888 and 1898 and no case of yellow fever developed from them. During the same period 450 non-immunes from Havana gave 13 cases in the quarantine stations. _The Yellow Fever Mosquito._—A knowledge of the life history of _Stegomyia calopus_ explains the epidemiology of yellow fever. This culicine species is widely distributed in the tropical and subtropical world, extending from 38° north to 38° south latitude. It is rarely found at a greater altitude than 3000 feet. Petropolis, a railway-connected suburb of Rio, has an altitude of 2300 feet, with cool nights, at times about 9°C, and a freedom from _Stegomyia_. Persons having occupation in Rio during the day but returning to Petropolis in the afternoon escape yellow fever. In this connection it is generally accepted that the female _Stegomyia_ only bites between 5 o’clock P.M. and midnight. While the first feeding may occur earlier in the day, all subsequent feedings, which alone could be infectious, occur late in the afternoon or at night. (Recent observations show _Stegomyia_ to bite in daytime,—not at night.) These views, which were advanced by Marchoux, would explain the apparent freedom from infection of those leaving infected areas by the early afternoon. Seidelin, however, claims that these mosquitoes will continue to bite in the day after numerous feedings of blood. It is recognized that railways are unimportant factors in transporting these mosquitoes, differing in this respect from ships which offer better conditions. The _Stegomyia_ is preëminently a house mosquito and a town mosquito. It is the domesticated one, while the malaria-transmitting ones are rural and feed in natural plant-containing bodies of water instead of the water in old tin cans, roof gutters, cisterns or other utensils surrounding the house which are preferred by the yellow fever mosquitoes. _Stegomyia_ seem to prefer water for breeding that is slightly tainted with sewage, although developing equally well in fresh water. They will also develop in brackish water. When once this mosquito takes up its residence in a certain room of a house it rarely leaves it and thus is explained the danger of occupying a room which has been occupied by a yellow fever patient. Then too, the warning sound, so characteristic of the approach of most mosquitoes, is not given by _Stegomyia_.