72 ESSAYS ON BACTERIOLOGY. 



Even though this obstacle to experiment remain, 

 there are other, though indirect, means of demon- 

 strating the specific relation of a given bacterium to a 

 disease. For instance, if it be shown that a certain 

 germ may be used to furnish a successful vaccine, a 

 defensive proteid, or immunizing blood serum, against 

 the disease from which it was obtained, the fact goes 

 far toward proving that germ to be the specific exciter 

 of that disease. Thus the road, blocked in one direc- 

 tion, opens up in another. 



As to the question whether each of the infectious 

 diseases has a single specific causative agent, the an- 

 swer may now be more satisfactory. There are some 

 maladies of which it seems to have been abundantly 

 shown that this is true; for instance, tuberculosis and 

 glanders. There are others, however, which have or- 

 dinarily been looked upon as specific diseases, of whicli 

 we now take a different view. Among these are the 

 septic and inflammatory affections "which do not pre- 

 sent such sharp and definite differential characters as 

 do those more specific infectious diseases which arc 

 caused by only a single species of micro-organism. 

 Some of these present clinical and pathological differ- 

 ences in accordance with their etiological differences, 

 whereas others do not. To this class belong acute ul- 

 cerative endocarditis, meningitis, broncho-pneumonia, 

 erysipelas, pyemia, septicemia, osteomyelitis, and pu- 

 erperal fever. Most of these names are simply col- 

 lective terms for different diseases which have already 



