42 



been ascertained that different kinds of organism 

 productive of different diseases exist, but through 

 the indefatigable researches of Pasteur and others 

 the distinguishing form and life history of certain 

 of these organisms have been clearly made out. 

 Placed under suitable conditions, it has been found 

 that they can be reared or cultivated artificially, 

 and one of the most marked and important 

 characters belonging to them is the enormous ex- 

 tent of self-propagating power they possess. This 

 accounts for the rapid spread that is observed to 

 take place of an infectious disease, if allowed 

 to progress without controlling measures being 

 brought to bear upon it. 



We have to deal, then, with something that 

 lives and grows by virtue of a power pertaining 

 to itself. Permit this living growth — this parasite 

 in fact — to become dispersed and to enter the 

 system of a living person, and presuming it has 

 lodged upon a soil supplying suitable conditions 

 for its development, it will thrive and multiply 

 and give rise to a series of phenomena, which the 

 physician has no power to arrest. Once the 



