CHAPTEK X. 



ACQUIRED IMMUNITY. 



Acquired immunity may be either active or pas- 

 sive: active when it arises as a consequence of 

 infection or artificial immunization (vaccination) ; 

 passive, when protective or curative serums are 

 injected. 



One who has recovered from scarlet fever, small- 

 pox, plague, typhoid fever, etc., becomes possessed 

 of lasting protection against subsequent attacks. On 

 the other hand, the immunity afforded by an at- 

 tack of certain other diseases usually is of shorter 

 duration: cholera, diphtheria, pneumonia, etc. So 

 far as known, the acquired protection is specific 

 in character : that is, a person who has had measles 

 may still have scarlet fever ; or an attack of cholera 

 does not protect against a later attack of t,yphoid. 



In a number of diseases one attack confers no 

 evident protection against a second: gonorrhea, 

 influenza, recurrent fever and malaria. Some dis- 

 eases may create a predisposition for recurrence: 

 erysipelas, influenza, diphtheria in some instances, 

 although a natural susceptibility of the individual 

 may explain repeated attacks. The mere fact of 

 recovery, however, is sufficient evidence of at least 

 a temporary immunity. It is evident, therefore, 

 that among the various infectious diseases different 

 grades of active immunity must be recognized. 



Certain chronic diseases are of particular inter- 

 est in this connection, as pointed out in Chapter 



