quent in the United States, in England, and in other countries where the value of vaccination is pretty generally recognized.
The influence of climate, and therefore of geographic distribution, upon the prevalence of certain diseases is due to its effect in increasing individual susceptibility to infection. Thus the susceptibility to influenza, to diphtheria, and to pneumonia is increased by exposure, leading to a sudden refrigeration of the body. These diseases are for this reason most prevalent in northern latitudes and during the seasons when by reason of exposure to sudden changes in the temperature there is the greatest liability to "catch cold."
It will be seen from what has already been said that the ætiology of infectious diseases does not depend alone upon exposure to infection—i. e., upon the presence of the specific infectious agent or germ, which is, however, an essential factor—but that the development of an attack may depend upon other factors which we may include under the general heads of (a) predisposing causes and (b) exciting causes. Predisposition may be either inherited or acquired. Thus the African race is especially liable to contract smallpox in its most virulent form, and the fair-skinned races of northern Europe are especially subject to fatal attacks of yellow fever. Again, certain families have a hereditary predisposition to pulmonary consumption, while others are especially liable to repeated attacks of smallpox in the same individual, etc. Youth constitutes a predisposition to certain diseases, the liability to attack being greatly diminished for scarlet fever and whooping-cough after adolescence and for tuberculosis after forty years of age. An acquired predisposition may be due to starvation or an inadequate diet as regards certain essential elements, to excessive fatigue or nervous exhaustion from any cause, to loss of blood, to alcoholic excesses, to insanitary surroundings, and in short to any of the causes which lower the vital resisting power of the individual. When such causes are general in their operation, or in times of famine, epidemics are likely to prevail, and the geographic range of these epidemics will coincide with the area in which the predisposing cause is effective.
As instances of the development of an attack from the direct action of an exciting cause (b), the specific germ being present, we may mention the effect of a recent debauch in causing an attack of yellow fever, of exposure to cold as the immediate cause of an attack of pneumonia or of influenza, of an attack of indigestion in developing a case of Asiatic cholera, of an injury to a joint as the exciting cause of a tubercular joint disease, etc.
What has already been said will show that the question of the geographic distribution of infectious diseases could hardly have been considered independently of questions relating to the ætiology or
