ACQUIRED IMMUNITY 77 



llallier and others, 06 had first to be scientifically refuted by Cohn, 

 Koch, and their pupils, before it could be assumed that a given in- 

 fectious disease was always the result of infection with a definite 

 and constant species of bacteria. The same applied to the specificity 

 of toxins and rational investigations into the reaction of the animal 

 body against bacterial poisons was not possible until the works of 

 Roux and Yersin on diphtheria and that of Kitasato on tetanus had 

 differentiated between the true, specific bacterial poisons and the 

 unspecific ptomains and "sepsins" of Selmi, E"encki, and Brieger. 



66 Hallier. Cited from Behring, "Bekampfung der Infektionskrank- 

 heiten," Leipzig, 1894. 



