The actual germ, microbe, or minute parasitic organism which causes yellow fever, and is carried by the fasciate gnat, has not yet been detected. Nevertheless, without seeing and isolating the microbe, the medical men of America (Sternberg, Finlay, Carroll, and others) have, by destroying the gnat and preventing its access to men--especially to patients already infected, and, therefore, certain to infect the gnats and cause them to spread the disease--practically made an end of yellow fever in many great cities of the New World, where it was only six years ago an ever-present horror, striking men down with a suddenness and with a deadliness which paralysed human activity. Here, as in other cases, intelligent appreciation of the results of science by a governor or a municipality has saved thousands of lives. On the other hand, in Rio de Janeiro, “the opposition encountered by the sanitary authorities of the city from political factions and the ridicule to which they were subjected by the local Press” were insuperable (I quote from an official report), and so a few more thousand lives were sacrificed before the master was recognised and the proffered safety accepted. In Vera Cruz, in New Orleans, and in Panama yellow fever has been reduced to a vanishing quantity by removing the pools and tanks in which the fasciate gnat can breed, and by making use of wire-gauze to prevent the access of mosquitoes to houses, bed-chambers, drains, and baths, and especially to prevent not only their access to, but their egress from, the rooms and beds of patients already infected with disease.