PARASITIC BACTERIA. 141 



of health these bacteria have little power of in- 

 juring him even when they do get into such 

 wounds, while at times of feeble vitality they 

 may do much more injury, and take the occasion 

 of any little cut or bruise to enter under the skin 

 and give rise to inflammation and pus. Some 

 people will develop slight abscesses or slight in- 

 flammations whenever the skin is bruised, while 

 with others such bruises or cuts heal at once 

 without trouble. Both are doubtless subject to 

 the same chance of infection, but the one resists, 

 while the other does not. In common parlance, 

 we say that such a tendency to abscesses indi- 

 cates a bad condition of the blood a phrase 

 which means nothing. Further, we find that the 

 same species of bacterium may have varying 

 powers of producing disease at different times. 

 Some species are universal inhabitants of the 

 alimentary canal and are ordinarily harmless, 

 while under other conditions of unknown char- 

 acter they invade the tissues and give rise to a 

 serious and perhaps fatal disease. We may thus 

 recognise some bacteria which may be compared 

 to foreign invaders, while others are domestic 

 enemies. The former, like the typhoid bacillus, 

 always produce trouble when they succeed in 

 entering the body and finding a foothold. The 

 latter, like the normal intestinal bacilli, are al- 

 ways present but commonly harmless, only under 

 special conditions becoming troublesome. All 

 this shows that there are other factors in deter- 

 mining the course of a disease, or even the exist- 

 ence of a disease, than the simple presence of a 

 peculiar species of pathogenic bacterium. 



From the facts just stated it will be evident 

 that any list of germ diseases will be rather un- 



