572 MICRO-ORGANISMS AND DISEASE [CHAP. 



recent years has it been possible to demonstrate experi- 

 mentally the intimate nature and causes of the immunity 

 and resistance acquired by a first attack. Salmon and 

 Theobald Smith, Beumer and Peiper, Ehrlich and Fraenkel, 

 Roux and Chamberland, Roux and Yersin, showed that an 

 animal acquires immunity against a particular infection not 

 only by Pasteur's method, i.e. by a first infection with miti- 

 gated or attenuated microbes — by a mild attack — but that 

 such immunity can be acquired also by previous injection 

 or injections of the ready-made specific toxins. Behring,i 

 Behring and Kitasato,^ Roux,^ and others followed this up 

 by showing by more exact methods (Diphtheria and Tetanus) 

 that a definite relation exists between the degree of resistance 

 acquired and the amount and virulence of the infecting 

 material (both microbes and specific toxin) used for the 

 antecedent injection or injections ; further that it is possible 

 to raise this resistance up to more or less complete immunity 

 by intermittent, repeated, and gradually increasing doses, 

 used in these antecedent injections, allowing the animal time 

 to recover completely before the next injection. 



Starting with a small dose or a mitigated virus —microbe 

 or toxin — the mitigation being achieved by heat, chemical 

 reagent, or method of cultivation — the first injections produce 

 slight reaction, if the animal is possessed at the outset of a 

 certain spontaneous resistance ; the reaction is greater, large 

 tumour in subcutaneous injections, constitutional disturbance 

 in most cases, if the animal is at the outset more susceptible. 

 The less the initial resistance of the animal or the greater or 

 more virulent the first dose, i.e. the greater the reaction on 

 the part of the animal, the greater as a rule is the resistance 



' Dcnlsche mcd. Woch. No. 49, i8go. 



'■^ Ibitl., No. 50, i8go. 



'^ Annales de P Institut Pasteur, September, 1894. 



