THE CAUSE OF INFECTIOUS DISEASE. 255 



dead, lifeless, organic or inorganic material. 

 Such relations are sometimes very easily traced, 

 but are often obscure and in other cases are 

 wanting. The parasitic organisms may ac- 

 cordingly be separated into obligatory para- 

 sites, facultative saprophytes, and facultative 

 parasites. 



In the group of obligatory parasites the de- 

 pendence upon processes of putrefaction, and 

 the ability to live at the cost and by the destruc- 

 tion of lifeless food material, have gradually 

 been completely lost, or at least such relations 

 have up to the present not been made out. In 

 this group may belong perhaps the yet undis- 

 covered germs of the so-called acute exanthe- 

 mata like smallpox, scarlet-fever and measles, 

 and also the germ already discovered in re- 

 lapsing fever. The facultative saprophytes 

 are those germs that we find as a rule living 

 as parasites, but which, under special conditions, 

 can maintain themselves also upon lifeless 

 material, and by breaking down this lifeless 

 nutrient substance are able to grow, multiply 

 and perpetuate the species. Such a sapro- 

 phytic condition has been brought about in the 

 case of the tubercle bacillus by Koch and 

 by Fischel, one of my pupils, and I was able 

 to show that this organism, which up to that 

 time had been called tubercle bacillus is only 



