THE CAUSE OF INFECTIOUS DISEASE. 257 



The organism may also be affected through 

 the removal of protective structures of the 

 body by the action of putrefactive poisons ; for 

 example, the intestinal epithelium may be 

 destroyed ; saprophytes may then enter into 

 the dead tissues, and may even penetrate still 

 farther into the body, as for instance into the 

 nearest lymph glands. The common bac- 

 terium of the large intestine, B. coli communis, 

 can do this. There are found, furthermore, 

 transition forms between the different groups 

 of parasitic microbes, so that it is evident that 

 we are not here dealing with rigid groups, but 

 only with a division which enables us to recog- 

 nize more easily the characters important from 

 a human standpoint. In the process of putre- 

 faction, then, a process which forms an abso- 

 lutely necessar} r link in the cyclical course of 

 matter, are found represented the fundamental 

 phenomena of parasitism out of which by de- 

 velopment and adaptation to living hosts the 

 various and more advanced stages of parasitism 

 have arisen. 



Putrefaction may exert in other respects an 

 important influence upon the excitation of dis- 

 ease ; volatile or soluble poisons of putrefaction 

 may weaken the living organism so that it can 

 be attacked more easily and successfully by 



the true parasites or by their toxins. The 

 17 



