SUSCEPTIBILITY TO INFECTION 25 



isolation, have been previously exempt from it, it is 

 apt to be exceptionally fatal. This is no doubt due 

 to the fact that there has been no opportunity for the 

 operation of the laws of natural selection, by " sur- 

 vival of the fittest." But under the operation of 

 these laws, in process of time, a certain degree of 

 race immunity is likely to be established. 



Individual susceptibility depends to some extent 

 upon age. As a rule, young animals are more sus- 

 ceptible to infection by inoculation than adults of 

 the same species. In the human race we recognise 

 certain diseases as especially liable to prevail among 

 children scarlet fever, whooping-cough, etc. It is 

 also known that the tendency to tubercular infection 

 diminishes with advancing years. Tubercular menin- 

 gitis and tubercular joint diseases are most common 

 in children, and pulmonary consumption in young 

 adults. Again, the susceptibility of individuals de- 

 pends to a considerable extent upon conditions re- 

 lating to their general ,health. Various depressing 

 agencies increase the susceptibility to infection. Most 

 prominent among these are insufficient food, insani- 

 tary surroundings, great fatigue, and mental worry 

 grief, fear, etc. The fact that pestilence and famine 

 are likely to go hand in hand has long been known. 

 Whether the prevailing epidemic be cholera, bubonic 

 plague, relapsing fever, typhus, or smallpox, the 



