(a) _Disease_

Disease, in the form of malaria, contributed to the fall of ancient Greece
and Rome. In the fourteenth century 25,000,000 people, one-quarter of the
population of Europe, were exterminated by plague, the "Black Death," and
in the sixteenth century smallpox depopulated Spanish America. Although
these particular diseases have lost much of their power owing to the
progress of medical science, we have no right to assume that disease in
general has been conquered by our civilisation, or that a new pestilence
may not appear. On the contrary, in 1805, a new disease, spotted fever,
appeared in Geneva, and within half a century had become endemic throughout
Europe and America. Of this fever during the Great War the late Sir William
Osler wrote: "In cerebro-spinal fever we may be witnessing the struggle of
a new disease to win a place among the great epidemics of the world." There
was a mystery about this disease, because, although unknown in the Arctic
Circle, it appeared in temperate climates during the coldest months of the
year. As I was able to prove in 1915, [8] it is a disease of civilisation.
I found that the causal organism was killed in thirty minutes by a
temperature of 62 deg. F. It was thus obvious that infection could never be
carried by cold air. But in overcrowded rooms where windows are closed, and
the temperature of warm, impure, saturated air was raised by the natural
heat of the body to 80 deg. F or over, the life of the microorganism,
expelled from the mouths of infected people during the act of coughing, was
prolonged. Infection is thus carried from one person to another by warm
currents of moving air, and at the same time resistance against the disease
is lowered. Cold air kills the organism, but cold weather favours the
disease. In that paradox the aetiology of cerebro-spinal fever became as
clear as the means of prevention. The story of spotted fever reveals the
forces of nature fighting against the disease at every turn, and implacably
opposed to its existence, while man alone, of his own will and folly,