ARTIFICIAL I.MMUNITY 549 



susceptible animals, or to act as a therapeutic agent for those 

 affected with the same disease. Diphtheria antitoxin is the 

 most striking example. 



§ 444. Method for artificially producing immunity. 

 The present knowledge of the subject shows that immunity 

 may be produced in several ways other than by causing the 

 individual to pass through an attack of the disease caused by 

 natural infection. These methods may be summarized as 

 follows : 



1. By inoculating the individual with a non-lethal dose 

 of a strong virus. This is practiced in immunizing cattle 

 against Texas fever, sheep pox and contagious pleuro- 

 pneumonia. 



2. By inoculating the individual with attenuated virus. 

 This is practical in anthrax, blackleg, chicken cholera, rouget, 

 and rabies and bubonic plague in man. 



3 By inoculating the individual with a vaccine consist- 

 ing of the virus of the disease modified by continual passage 

 through another species of animal, as vaccine for small pox. 



4. By the injection of toxins. In practice this is used 

 for immunizing animals such as horses against the virus ot 

 the diseases for the purpose of procuring antitoxin from their 

 blood, as in diphtheria and tetanus. 



5. By the injection of antitoxins. These are used to 

 immunize animals against toxins, and children against natural 

 infection, as in diphtheria. 



i; 445. Active and passive immunity. In methods 

 producing artificial immunity, most of them require the intro- 

 duction into the body of the living virus or the injection of the 

 products (toxins or heated cultures) of the living virus in 

 repeated doses and constantly increasing quantities. Immunity 

 produced by any of these procedures or by the recovery from 

 an attack of an infectious disease is called active immunity. 

 When immunity is produced by the injection of the serum 

 (antitoxin) of animals already immune it is called passive 

 immunity. 



