CHAPTER XII 



ACQUIRED IMMUNITY 



IT is well known that in certain infectious diseases a 

 single attack protects the individual from subse- 

 quent attacks. In some cases such protection lasts 

 during life, while in others it is more or less temporary. 



The protection afforded by an attack not only 

 varies in different diseases, but in the same disease 

 differs greatly in individual cases. Thus second or 

 even third attacks of smallpox occasionally occur, 

 although, as a rule, a single attack is protective. 



In certain diseases second attacks are not infre- 

 quent. This is true of pneumonia, Asiatic cholera, 

 diphtheria, and especially of influenza. But there is 

 usually a considerable interval between two attacks 

 of any of these diseases, and the inference is that 

 temporary immunity results from each attack. In 

 the malarial fevers, which are due to infection by a 

 blood parasite of a different class, no immunity is 

 afforded by an attack of the disease, in its usual form 



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