Part IV 



VACCINES IN THE PROPHYLAXIS 

 OF DISEASE 



Chapter XIV 



PROPHYLACTIC VACCINES 



SMALLPOX 



Vaccines usually employed in the practice of 

 medicine may be divided into three classes: (1) 

 attenuated vaccines (viruses), (2) killed bac- 

 terial vaccines of Wright, and (3) pollen vac- 

 cine. An attenuated vaccine is one in which the 

 virulence has been decreased in some manner, 

 usually by exposure to temperatures unfavor- 

 able to the growth of the microorganism or by 

 passage through animals. For example, the 

 virus of rabies was attenuated by passing it 

 through a series of fifty rabbits which rendered 

 the virus of increased virulence for the rabbit 

 but of decreased virulence for man. Bacterial 

 vaccines are usually suspensions in physiologic 



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