This chapter presents the history of yellow fever in Havana, Cuba, as given by General Gorgas. Yellow fever illustrates perhaps better than any other disease how, without accurate knowledge of the natural history of a disease, humanity may flounder for decades or even centuries in helpless blundering; being dependent for protection during this period of darkness on the natural limitations to the spread of the disease afforded by climate and by infrequency of human communications. The main investigation which cleared up the problem of yellow fever was inspired by the investigations of Manson and Ross. Yellow fever was so serious to the American troops that Havana might have had to be evacuated by them if the war with Spain had not ended speedily. Anti-malarial measures coincide with those against yellow fever, with the exception of treatment by quinine, and the difference in action based on the fact that the stegomyia is a domestic, the anopheles a chiefly non-domestic breeder.