It requires continuously warm weather for the yellow-fever mosquito to breed in sufficient numbers to propagate yellow fever; therefore, this disease never became endemic in the United States. I mean by endemic, existing all the year round and over a number of years. The frosts of winter, wherever they occur, either destroy all the yellow-fever mosquitoes, or reduce their number below the point at which yellow fever could be propagated. It was known in the United States that yellow fever was always brought somewhere from the littoral of either the Gulf of Mexico, or the Caribbean Sea, and the city of Havana, located on the northern coast of the island of Cuba, was known to be the center of this endemic area. Yellow fever in 1898 was looked upon as the example of a filth disease, par excellence, and it was thought that if Havana were put in a proper state of cleanliness, it might cease to be the great point of infection for the United States. It was known that yellow fever had existed in the city of Havana continuously for one hundred and fifty years. It is interesting to note that the endemic infection of Havana occurred in 1762, when Havana was besieged and captured by American troops. I say American troops, because the expedition was largely composed of men from the present United States, then colonies of Great Britain. It is also interesting to note that this infection was supposed to have been brought by a vessel from Vera Cruz. Yellow fever peculiarly affects shipping, and time and again ships in the harbor of Havana have had every living soul of their crew die from this disease, and these vessels would have to lie there for months until another crew could be obtained. When we went to Havana in 1898 we knew no more of the sanitation of yellow fever than we had known a century before. The army which went to Santiago suffered as severely from yellow fever and other tropical diseases as any military expedition into the tropics had suffered before that time, and its death rate, had it remained, would have been just as high as was that of the French army of similar size, which was exterminated in the island of Haiti just one hundred years before.