CHAPTER IV. 



IMMUNITY. 



By immunity, is understood the inherent power of a living body 

 to successfully withstand the invasion of infective agents, e.g., bac- 

 teria, or such deleterious and toxic substances as toxins, drugs, com- 

 plex poisonous albumins, snake venom, foreign blood sera, etc. ^ 



The following tables will, perhaps, be helpful in the study of the 

 subject. 



i. Immunity 



2. Immunity 



It is a well known fact that one attack of an infectious disease 

 generally protects an individual against a subsequent attack. It 

 has also been known for centuries that the human system, by first 

 taking very small doses, and gradually increasing them, can be so 

 accustomed to poison, that large, and otherwise deadly quantities 

 may be taken at one time with impunity. Among the poisonous 

 substances to which men can accustom themselves are: tobacco, 

 morphia, arsenic, and alcohol. Animals treated in a like manner 

 also become immunized to powerful toxins, snake venom, etc. 



Natural Immunity. The hog is immune to snake venom; the 

 chicken to tetanus. Man is immune to hog, or chicken cholera. 

 The negro is not so susceptible to yellow fever as is the white. Ani- 

 mals cannot be infected with scarlet fever, malaria, and measles. 



