IMMUNITY 



229 



(these apparently greater susceptibilities may be due in part 

 to the fact that most of the older individuals have had the 

 diseases when young and are immune for this reason). Ani- 

 mals weakened by hunger or thirst succumb to infection 

 more readily. Frogs and chickens are immune to tetanus, 

 but if the former be put in water and warmed up to and 

 kept at about 37°, and the latter be chilled for several hours 

 in ice-water, then each may be infected. Pneumonia fre- 

 quently follows exposure to cold. The immune rat may be 

 given anthrax if first he is made to run in a "squirrel cage" 

 until exhausted. Alcoholics are 'far less resistant to infec- 

 tion than temperate individuals. "Worry," mental anguish, 

 tend to predispose to infection. 



The following outlines summarize the' different classifi- 

 cations of immunity so far as mammals are concerned for 

 the purposes' of discussion: 



Immunity. 



I. Natural ' 



A. Congenital 



(■■ 



Inherited through the germ cell or cells, 

 (a) By having the 



[ 2. Acquired in utero. 

 B. Acquired by having the disease. 



disease in utero. 

 (b) By absorption 

 of immune sub- 

 stances from the 

 mother. 



II. Artificial — acquired through human agency by: 



1. Introduction of the organism or its products. 



2. Introduction of the blood serum of an immune animal. 



Immunity. 



I. Active — due to the introduction of the organism or due to the intro- 

 duction of the products of the organism. 



A. Naturally by having the disease. 



B. Artificially. 



1. By introducing the organism: 



1. Passage through another animal. 



(a) Alive and virulent. 

 (6) Alive and virulence 



reduced by 

 (c) Dead. 



2. Drying. 



3. Growing at a higher temperature. 



4. Heating the cultures. 



5. Treating with chemicals. 



6. Sensitizing. 



7. Cultivation on artificial media. 

 2. By introducing the products of the organism. 



II. Passive — due to the introduction of the blood serum of an actively 

 immunized animal. 



