In sparsely populated districts, in which the course of the disease could be more accurately observed, it could readily be seen that influenza spread gradually in all directions from the district which was known to have been first affected-that is to say, it radiated from a common center. On the other hand, in districts in which communication was more general, the leaping or vaulting character of the disease was often very marked. One need only remember that in the case of the pestilence which spread from Russia, Paris and
London were affected much earlier than innumerable cities and many provinces of Germany, even than those lying close to the Russian frontier.
These observations settle the old controversy of historians as to whether influenza is invariably distributed in a leaping or in a radiating manner. As a matter of fact, both modes occur according to the manner and means of intercourse.
But few places, in Europe especially, remained unattacked by influenza in 18S9 and 1S90. They were without exception districts not in ordinary communication with the outside world, such as villages, hamlets, deserts, desolate places, coasts, forest valleys, peaks of hills, and mountains. The following places have been definitely shown to have been exempt: Isle of Man, several West India Islands (Bahamas, Grenada, St. Lucia), the farther British Honduras, British New Guinea, and the Seychelles.
Upon the Santis (observatory, 2504 meters) even- one remained well, "because during the whole time of the epidemic no one descended into or ascended from the valley."
Localities which were temporarily cut off from outside communication were affected by the influenza at correspondingly later dates.
Upon the island of Borkum, which, on account of the frost, was isolated from the mainland from Christmas to the fifth of January, the disease showed itself on the eighth of January, four days after the arrival of the first vessel.
In several localities of the Weser marsh (Kreis Achim), which, on account of flood, was cut off for weeks, the disease arose at a correspondingly later period.
Vladivostok and the island of Sakhalin were not attacked until the spring of 1890, after communication by sea was again established.
If we inquire what are the reasons why influenza, as compared with other contagious diseases, spreads with such rapidity over land and sea and brings about such universal infection, they would appear to be as follows: The marked virulence of the contagion; the ease with which the germs are conveyed from their original seat in the mucous membranes of the respiratory apparatus to outside and all around by coughing,s sneezing, and expectoration; the enormous number of slightly affected persons who carry on their vocation and intercourse with their fellowmen, and who travel about to all points of the compass without hindrance; the probable longevity of the germs in convalescents; the brief period of incubation,-one to two days,-on account of which the number of those affected grows to an enormous extent within a few days; the almost equal susceptibility of all people, every age, and vocation; the probable transmission of the germ by healthy individuals and by merchandise of various kinds, and for short distances also through the air. We are conditional supporters of the doctrine regarding the distribution of influenza germs by means of the air.