258 BACTERIOLOGY. 



mere presence and growth of certain sapro- 

 phytes acts in such a way that disease germs 

 following in their wake can get lodgment upon 

 man the more readily, while on the other hand 

 other saprophytes may hinder the lodgment of 

 pathogenic organisms. Among the various 

 effects produced by saprophytic microbes out- 

 side and inside a living host must be included 

 those that either favor or hinder the lodgment 

 and action of disease germs. That is to say, 

 these microbes act upon the disposition toward 

 disease. They present therefore only individ- 

 ual cases, albeit particularly difficult to evalu- 

 ate, of external relations or conditions which 

 may now exalt an existing disposition to dis- 

 ease, now diminish it or remove it altogether. 

 This explanation should make it no longer 

 difficult for the reader to understand the very 

 various modes of action of disease-germs in 

 man, since every possibility of action has de- 

 veloped out of two activities already manifested 

 in the process of putrefaction, namely out of 

 the formation of poisons by bacteria, and out of 

 bacterial growth and multiplication. At one 

 extreme, therefore, we find a kind of parasitic 

 action in which not the bacteria themselves 

 but the poisons formed by them and absorbed 

 into the circulation are the more important 

 factor, while upon the other side stand those 



