rarely occurs inland, but follows water-courses and lines of ocean-travel, and so usually appears in commercial cities, and begins its march at the wharves. It is uncommon in elevated regions, and 2,500 feet is commonly regarded as its altitudinal limit, but it has been known to occur at Newcastle, Jamaica, at a height of 4,000 feet, and if the belief be true that ancient Mexico was visited by it under the name of matlazahuatl, then it has been epidemic at a height of between 7,000 and 8,000 feet above the sea.
There has been no severe epidemic of this disease in New York since 1822, but it breaks out on our Gulf and South Atlantic coasts at intervals, with no appearance of periodicity. It first appeared in New Orleans in 1796, and has often been epidemic there since. The most fatal epidemic was that of 1853, when the deaths were variously stated at from 8,000 to 10,000, or about eight times as many as have occurred there during the present summer, though the population was only half as large. It is the common impression that New Orleans was saved from the disease during its occupation by Federal troops in 1862-'65, because the city was put in such excellent sanitary condition; but Dr. Nott calls attention to the fact that there are often periods of exemption from the disease in all places visited by it, and that in New Orleans in 1859 there were only 91 deaths from yellow fever, in 1860 only fifteen, and in 1861 not a single one; while in 1863 Dr. Harris says there were nearly 100 cases of the disease, with two officially recorded deaths, and in 1864 more than 200, with 57 deaths.
Yellow fever occupies a singular position between the contagious and non-contagious diseases. The poison is not, like that of small-pox, directly communicable from a sick person to a well one; but, although the emanations of the sick are connected with the spread of the disease, they seem to require an appropriate nidus in which to germinate and develop. This nidus must be warm and moist, and there the germs, whatever they are, lie and grow, or, in some way develop until they are able to migrate. The germs are portable, and may be conveyed in baggage or merchandise (fomites) for hundreds or thousands of miles. If not so conveyed, its progress is very slow. In 1822 in New York, when it gained a foothold in Rector Street, it appeared to travel about forty feet a day until killed by the frost. It often leaves a house or a block intact, going around it and attacking those beyond, with no assignable reason. A thin board partition seems to have stopped it on Governor's Island in 1856, and an instance is related where it attacked the sailors in all the berths on one side of a ship before crossing to the other. Such apparent vagaries are, in the present state of our knowledge, inexplicable.