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°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°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, harbours infection and creates the only conditions under which the malady can appear. For example, during two consecutive winters cerebro-spinal fever had appeared in barracks capable of housing 2,000 men. A simple and effective method of ventilation was then introduced. From that day to this not a single case of cerebro-spinal fever has occurred in these barracks, although there have been outbreaks of this disease in the town in which the barracks are situated. There are many other diseases peculiar to civilisation, and concerning the wherefore and the why an apposite passage occurs in the works of Sir William Gull.