TYPES OF IMMUNITY 145 



(a) An attack of the disease ; 



(6) Introduction of vaccine consisting of the living causal 

 agent in a modified form ; 



(c) Introduction of a vaccine consisting of dead organisms ; 



(d) Introduction of toxin. 



An Attack of the Disease. The degree and duration of immu- 

 nity following an infection varies greatly according to the disease. 

 Immunity following smallpox, yellow fever, measles, scarlet 

 fever, typhoid fever, whooping cough, typhus fever, chicken pox, 

 and mumps is generally lasting ; in a few of the diseases, however, 

 second attacks have been known to occur. Certain other diseases, 

 such as pneumonia and erysipelas, seem to leave the individual 

 more susceptible to a second attack ; yet in these infections there 

 must be a certain amount of immunity, even though it is of short 

 duration, or the patient would not recover. 



Introduction of the Modified Causal Agent. Apart from an 

 actual attack of the disease this method of imitating nature pro- 

 duces the highest and most lasting degree of immunity. Edward 

 Jenner in 1798 established the fact when he succeeded in demon- 

 strating that vaccination with material from cowpox protected 

 an individual against smallpox. Eighty years later Pasteur 

 applied Jenner's principle to other forms of disease. About 1888 

 the chickens in the neighborhood of Paris were being destroyed in 

 great numbers by a virulent intestinal infection. Pasteur isolated 

 an organism which he found to be the cause of the disease and which 

 when injected into healthy chickens produced all the characteris- 

 tic symptoms. Then he discovered that by prolonged cultivation 

 on artificial medium the bacillus could be so attenuated that when 

 injected into chickens no harm resulted, and, what was of much 

 greater importance, these same chickens when inoculated with 

 freshly isolated virulent organisms were found to be immune. 



Pasteur then turned his attention to the study of anthrax. 

 He found, however, that the methods applied to immunize chickens 

 against cholera were not applicable in this case. Prolonged culti- 

 vation produced spore formation and not attenuated cultures. 



