If, therefore, the streets of a city are unpaved and
ungraded and there are open spaces where water may accumulate in pools,
as well as open cesspools to serve as breeding places for _Culex
fasciatus_, the city will present conditions more favorable for the
propagation of yellow fever than it would if well paved and drained and
sewered.
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.
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