THE CONTEST AGAINST INFECTION. 



IT has become evident that bacteriology is now pro- 

 gressing chiefly in the direction of discovering the 

 channels through which the self-limitation of and im- 

 munity from infectious diseases is brought about. As 

 stated heretofore, enough has been learned to estab- 

 lish the principle that the effects of bacteria are pro- 

 duced by chemical substances contained in their bodies 

 or resulting from the growth of these bacteria. Suffi- 

 cient also has been learned to show that self -limitation 

 and immunity are in some way the outcome of such 

 substances. The question, therefore, presents itself : 

 What is immunity, and through what channels and 

 by what agents is it brought about? 



xVt the outset we must recognize two forms of im- 

 munity: first, natural immunity, by which is meant a 

 natural insusceptibility of certain animals to certain 

 diseases; second, acquired immunity. 



Acquired immunity may be brought about in three 

 ways: First, by an attack of the disease; second, by 

 inoculation with an allied disease or with the disease 

 modified by passing the virus through another animal; 



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