elating 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