BACTERIAL VACCINES 171 



In the past few years it has become more and 

 more apparent that the limitations of serum therapy, 

 at least in the great majority of infectious diseases, 

 are at present almost insuperable. Attention was 

 therefore again turned to treatment by active 

 immunization. It was perhaps only natural, in 

 view of his discoveries in fermentation, that Pasteur 

 should have believed that the production of immu- 

 nity required the action of the living virus. He 

 therefore vigorously combated the idea that im- 

 munity could be brought about by means of dead 

 virus, or of lifeless products of growth of the virus. 

 Touissant, as far back as 1880, had held out for 

 the latter possibility, but the imperfections of 

 his technique were such that his views were not 

 accepted. To Salmon and Smith of this country 

 belongs the honor of first clearly demonstrating 

 the possibility of immunization w r ith dead cul- 

 tures. 



Methods of Active Immunization. Active im- 

 munization can be carried out in several ways: 



(i) By means of living cultures of the virus. 

 Usually the cultures are attenuated, but there are 

 some exceptions. 



A number of different procedures may be employed 

 to attenuate the virus. Thus, by drying, as is done 

 with rabies virus in the Pasteur treatment; or by 

 growing the virus at a temperature unsuited for the 

 development of virulence, as is done in the case of 



