PARASITIC BACTERIA. 139 



man disease ; and, finally, to reproduce the disease 

 in healthy animals by inoculating them with the 

 bacterium. All of these steps of proof present 

 difficulties, but especially the last one. In the 

 study of animals it is comparatively easy to re- 

 produce a disease by inoculation. But experi- 

 ments upon man are commonly impossible, and 

 in the case of human diseases it is frequently 

 very difficult or impossible to obtain the final 

 test of the matter. After finding a specific bac- 

 terium associated with a disease, it is usually pos- 

 sible to experiment with it further upon animals 

 only. But some human diseases do not attack 

 animals, and in the case of diseases that may be 

 given to animals it is frequently uncertain wheth- 

 er the disease produced in the animal by such in- 

 oculation is identical with the human disease in 

 question, owing to the difference of symptoms in 

 the different animals. As a consequence, the proof 

 of the germ nature of different diseases varies all 

 the way from absolute demonstration to mere 

 suspicion. To give a complete and correct list 

 of the diseases caused by bacteria, or to give a 

 list of the bacteria species pathogenic to man, is 

 therefore at present impossible. 



The difficulty of giving such a list is rendered 

 greater from the fact that we have in recent years 

 learned that the same species of pathogenic bac- 

 terium may produce different results under differ- 

 ent conditions. When the subject of germ dis- 

 ease was first studied and the connection between 

 bacteria and disease was first demonstrated, it 

 was thought that each particular species of 

 pathogenic bacteria produced a single definite 

 disease ; and conversely, each germ disease was 

 supposed to have its own definite species of bac- 



