longer sought directly in meteorological conditions, but in the effects, more or less direct, of these conditions upon the microorganisms which are the specific cause of the disease. Atmospheric conditions may help or may retard the development of the microorganism, and may strengthen or weaken the individuals' power of resistance. Winds used to be regarded as the chief agents in spreading epidemics: now it is known that disease can not be carried far by winds, for the microorganisms do not long maintain their power in the free air and under the sun. Eain has been supposed directly to control the distribution of disease: now we believe that precipitation acts only indirectly, through drinking-water, or through its control over the dust in the air. Dust from dry soil, and from city streets, may contain the germs of infectious diseases, and aggravate affections of the respiratory organs.
Geographical Distribution of Disease.—The scheme of classifying disease geographically, on a broad climatic basis, is attractive, but not very satisfactory. For, on the one hand, many diseases are practically universal in extent, showing great independence of climate, and, on the other, the history of many diseases is still in the making. In spite of this complexity, however, certain broad statements may be made, useful in enabling the layman properly to coordinate his ideas on the subject, and fairly accurate within reasonable limits.
Tropics: General Physiological Effects.—Tropical monotony of heat is associated with high relative humidity, except over deserts and in dry seasons. The air is therefore muggy and oppressive. This "hot-house air" has an enervating effect. Energetic physical and mental action are often difficult, or even impossible. The tonic effect of a cold winter is lacking. These conditions have certain fairly well-established physiological effects, which, combined with less power to do work, greater fatigue from work, and lowered vitality, render the body less able to resist disease.
Hygiene in the Tropics.—Under the peculiar conditions of tropical climates, the resident who comes from a cooler latitude should take special precautions regarding his mode of life and personal hygiene. A rational, temperate life, especially the avoidance of alcoholic excess; regular exercise; non-fat-producing food; clothing suited to the climate; all possible sanitary precautions; protection against mosquitoes; frequent change of climate by returning to cooler latitudes—all these are important. It seems like a contradiction, but it is a fact, that the danger of becoming chilled in the tropics is very great and must be carefully guarded against. General Wolseley is reported to have said of the tropics, "not to get cold is to avoid almost certainly all the causes of disease," and a recent writer has well said that these words should be inscribed on the walls of all barracks in the tropics. The situation may be summed up in this rule: "Respect the sun, and
