The question whether yellow fever may be transmitted by any other species of mosquito than Culex fasciatus has not been determined.  Facts relating to the propagation of the disease indicate that the mosquito which serves as an intermediate host for the yellow fever germ has a somewhat restricted geographical range and is to be found especially upon the seacoast and the margins of rivers in the so-called “yellow fever zone.”  While occasional epidemics have occurred upon the southwest coast of the Iberian Peninsula, the disease, as an epidemic, is unknown elsewhere in Europe, and there is no evidence that it has ever invaded the great and populous continent of Asia.  In Africa it is limited to the west coast.  In North America, although it has occasionally prevailed as an epidemic in every one of our seaport cities as far north as Boston, and in the Mississippi Valley as far north as St. Louis, it has never established itself as an epidemic disease within the limits of the United States.  Vera Cruz, and probably other points on the Gulf coast of Mexico, are, however, at the present time, endemic foci of the disease.  In South America it has prevailed as an epidemic at all of the seaports on the Gulf and Atlantic coasts, as far south as Montevideo and Buenos Aires, and on the Pacific along the coast of Peru.

The region in which the disease has had the greatest and most frequent prevalence is bounded by the shores of the Gulf of Mexico, and includes the West India islands.  Within the past few years yellow fever has been carried to the west coast of North America, and has prevailed as an epidemic as far north as the Mexican port of Guaymas, on the Gulf of California.

It must be supposed that Culex fasciatus is only found where yellow fever prevails.  The propagation of the disease depends upon the introduction of an infected individual to a locality where this mosquito is found, at a season of the year when it is active.  Owing to the short period of incubation (five days or less), the brief duration of the disease and especially of the period during which the infectious agent (germ) is found in the blood, it is evident that ships sailing from infected ports, upon which cases of yellow fever develop, are not likely to introduce the disease to distant seaports.  The continuance of an epidemic on shipboard, as on the land, must depend upon the presence of infected mosquitoes and of nonimmune individuals.  Under these conditions we can readily understand why the disease should not be carried from the West Indies or from South America to the Mediterranean, to the east coast of Africa, or to Asiatic seaport cities.  On the other hand, if the disease could be transmitted by infected clothing, bedding, etc., there seems no good reason why it should not have been carried to these distant localities long ago.