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the clinical picture that sufficient time is not available to stimulate 

 the body cells to active immunization. If the period of incubation 

 of a given malady is sufficiently long, so as to permit of the active 

 mobilization of the defensive forces before symptoms of disease 

 actually develop, attempts at active immunization would, of course, 

 be indicated, and, as shown in our prophylactic treatment of rabies, 

 may give rise to excellent results. Chronic infections further would 

 theoretically, at least, lend themselves to treatment of this order, 

 while in the acute maladies, for the reasons just indicated, we can 

 only exceptionally hope to exercise a favorable influence upon the 

 course of the disease. In combination with the use of antitoxic 

 or bacteriolytic sera, however, it might even then be tried. 



As the basis of all attempts at active immunization, we might 

 very appropriately take the dictum: that there can be no protection 

 without infection, bearing in mind, however, that "infection" is not 

 synonymous with "disease," that "infection" does not imply a 

 "virulent" infection, and that, immunologically speaking, the paren- 

 teral introduction of the killed pathogenic agent even may be equiva- 

 lent in its effects to infection. That infection with living virulent 

 organisms may result in protection has, of course, been recognized 

 as long as we have had knowledge of the etiologic connection of 

 microorganisms with the infectious diseases, but the discovery that 

 the same result can be achieved in many instances without the 

 production of any malady of moment, through the use of organisms 

 whose virulence has been artificially diminished, and, as I have 

 already indicated, even with organisms that are dead, is one of the 

 greatest triumphs of modern medicine. The various methods that 

 are employed to this end have already been briefly considered in a 

 previous chapter, and will be taken up in greater detail in connection 

 with the different infections against which active immunization is 

 employed. 



SMALLPOX 



It is an interesting coincidence that the principle just stated, viz., 

 the possibility of producing active immunity by the use of organisms 

 that have been attenuated in their virulence, was unconsciously 

 employed by the earliest workers in this field. 



When and how the discovery was made that the virulence of small- 



