292 POPULAR SCIENCE MONTHLY. 



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 distribu- 

 tion, upon the prevalence of certain diseases is due to its effect in 

 increasing individual susceptibility to infection. Thus the suscepti- 

 bility to influenza, to diphtheria, and to pneumonia is increased by 

 exposure, leading to a sudden refrigeration of the body. These dis- 

 eases are for this reason most prevalent in northern latitudes and dur- 

 ing 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 aetiology 

 of infectious diseases does not depend alone upon exposure to infec- 

 tion i. e., upon the presence of the specific infectious agent or 

 germ, which is, however, an essential factor but that the develop- 

 ment 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, cer- 

 tain families have a hereditary predisposition to pulmonary consump- 

 tion, while others are especially liable to repeated attacks of small- 

 pox 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 tubercu- 

 losis 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 geo- 

 graphic 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 (&), 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 develop- 

 ing 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 aetiology or 



