atmospheric conditions an explanation of the occurrence of disease. Many fairly obvious facts naturally point to some relation of cause and effect in this matter. Some diseases are found principally in warmer climates; others seem to prefer the colder. Some are usually more active in the warmer, or the drier months; others have shown the contrary relation. High altitudes are free from some of the diseases which prevail near sea-level, and have certain favorable climatic characteristics long recognized in the treatment of disease. In the case of other diseases, again, altitude has no effect. Dry climates, especially deserts, whose air is usually exceptionally pure and aseptic, are generally healthful, and are beneficial in many cases where mountain climates are too stimulating. The climates within forested areas have proved especially favorable in cases of phthisis. Ocean air, pure and dust-free, with its saline constituents and equability of temperature, is beneficial to most persons as a moderate tonic and as a restorative in many illnesses. Winds are active ventilating and purifying agents where population is congested. Fogs and clouds, by cutting off sunlight, weaken one of the best agents in promoting health, for sunlight, in the words of Dr. Sternberg, is "one of the most potent and one of the cheapest agents for the destruction of pathogenic bacteria." In London, a higher death-rate follows a long fog, but this may result from the lower temperature during the fog, and not from any direct effect of the fog itself.
A Complex Subject.—Facts like these naturally prejudice one in favor of a causal connection between atmospheric conditions and disease. Nevertheless, such studies have often led to very contradictory conclusions. Some of the difficulty arises from untrustworthy statistics, but most of the disagreement comes from the fact that not only may each of the different weather elements have some effect in the production of the disease, but so many other factors are concerned in the matter that confusion and contradiction in the conclusions reached are inevitable. Sanitation, food, water, habits, altitude, character and moisture of the soil, race, traffic and other controls, serve to complicate the problem. In most studies of climate and health some, or even many, of these factors have not received attention. Overcrowding under unhygienic conditions, especially indoors during cold weather, and traffic by rail, steamship, caravan or on foot, are often more important than climate. The frequent escape of mountain, of desert and of polar peoples from epidemics is to be attributed in most cases to the smaller chance of importing disease because of little intercourse with the outside world, and of spreading it, when imported, because of the scattered population. It may be noted, however, that crowding indoors in winter, and the sparseness of population just referred to, are themselves climatically controlled.
Climate, Microorganisms and Disease.—The cause of disease is no
