EVOLUTION OF DISEASE. 



PART I. 



CHAPTER I. 



THEORIES OF IMMUNITY OF METCHNIKOFF 

 AND EHRLICH. 



Experimental Researches Inspired by These Theories./ 



MEDICAL science has for its object to teach us^why and 

 how an organism becomes diseased, what constitutes this 

 disease, and how cure is effected. It is only by knowing how 

 to answer these three questions that it is possible to treat 

 disease successfully either by preventing its occurrence or 

 by assisting the organism in recovering from it. Up to the 

 time of Pasteur we knew as pathogenic agents only poisons 

 as such; chemically crystallizable products and venins, all 

 of unknown composition, but which with the crystalloids 

 have this fact in common, that the diseases of which they 

 are the cause are non-contagious. 



With the discovery of bacteria we began to know the origin 

 of contagious diseases and it was recognized at about the 

 same time, that if a certain microbe was the primary cause 

 of the disease it could act only by means of its soluble secre- 

 tions or by products which .were allowed to escape at its 

 death; that is to say, by poisons of an unknown composition 

 analogous to the venins. But although we know the cause of 

 disease, we do not know the mechanism by which the patho- 

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