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Rh endemic. Until recently it was always to be found in Havana. Formerly the Brazils enjoyed an absolute immunity; but from 1849, when yellow fever was introduced for the first time into Bahia by a ship from New Orleans, it was practically endemic in the large cities until the institution of anti-stegomyia measures recently. At Rio de Janeiro, although in some years the cases were few, it was never entirely absent. It is still very common in Manaos. In such places as New Orleans, Charleston, Monte Video, and Buenos Ayres, although now and again epidemics of great severity break out, several years may pass without its appearing. Some of these epidemic visitations bring a heavy death-bill; thus, in New Orleans, in 1853, 7,970 people died of yellow fever; in 1867, 3,093. In Rio, in 1850, it claimed 4,160 victims; in 1852, 1,943; and in 1886, 1,397. In Havana the annual mortality from this cause used to range from 500 to 1,600 or over.

Epidemiology.— Influence of atmospheric temperature.— The histories of these and other epidemics show that the virus of yellow fever can be transported from one place to another, and that for its development in epidemic form it requires an atmospheric temperature of over 75° F. It ceases to spread when the thermometer sinks below this point, and it stops abruptly as an epidemic when the freezing-point is reached. Dampness favours yellow fever; it is therefore most prone to occur and to spread during the rainy season.

Usually a sea-coast disease.— The favourite haunts of the disease are the sea-coast towns, the banks of rivers, and flat delta country. Rarely does it pass far inland, or ascend high ground. Still, there have been exceptions to this general rule; for yellow fever has been found far inland, and at a considerable elevation (Sao Paulo, Brazil, 2,500 feet; Newcastle, Jamaica, 3,000-4,000 feet; Cuzco, Peru, 9,000-10,000 feet). Villages are seldom affected; nor does the disease readily spread if introduced into rural localities. In spreading inland it follows the lines of communication railways, canals, navigable rivers.