way of the theory of exposure as a cause of pneumonia. One of the
classic ones was that, during Napoleon's frightful retreat from Moscow
in the dead of winter, while his wretched soldiers died by thousands of
frost-bite and starvation, exceedingly little pneumonia developed among
them. Another was that, as we have already seen with colds, instead of
being commoner and more frequent in the extreme Northern climate and on
the borders of the Arctic Zone, pneumonia is almost unknown there. Of
course, given the presence of the germ, prolonged exposure to cold may
depress the vital powers sufficiently to permit an attack to develop.

Again, the ages at which pneumonia is both most common and most deadly,
namely, under five and over sixty-five, are precisely those at which
this feature of exposure to the weather plays the most insignificant
part. Last and most conclusive of all, since definite statistics have
begun to be kept upon a large scale, pneumonia has been found to be
emphatically a disease of cities, instead of country districts. Even
under the favorable conditions existing in the United States, for
instance, the death-rate per hundred thousand living, according to the
last census, was in the cities two hundred and thirty-three, and for the
country districts one hundred and thirty-five,--in other words, nearly
seventy per cent greater in city populations.

How, then, did the impression become so widely spread and so firmly
rooted that pneumonia is chiefly due to exposure? Two things, I think,
will explain most of this. One is, that the disease is most common in
the winter-time, the other, that like all febrile diseases it most
frequently begins with sensations of chilliness, varying all the way
from a light shiver to a violent chill, or _rigor_. The savage,
bone-freezing, teeth-rattling chill which ushers in an attack of
pneumonia is one of the most striking characteristics of the disease,
and occurs in twenty-five to fifty per cent of all cases.

Its chief occurrence in the winter-time is an equally well-known and
undisputed fact, and it has been for centuries set down in medical works
as one of the diseases chiefly due to changes in temperature, humidity,
and directions of the wind. Years of research have been expended in
order to trace the relations between the different factors in the
weather and the occurrence of pneumonia, and volumes, yes, whole
libraries, published, pointing out how each one of these factors