CHAPTER IV. 

 BACTERIAL POISONS. 



IT has been pointed out in Chapter I that the terms infection and 

 infectious disease cannot be used synonymously. The existence of 

 an infectious disease itself implies the existence of an infection, but 

 infection may exist in the absence of any symptoms which denote 

 disease. In the ordinary trypanosomiasis of rats, for example, there 

 is nothing to suggest that the infected animal is in any way delete- 

 riously affected by the presence of the parasite. There is virtually a 

 symbiosis between the two, from which the host does not derive 

 any evident benefit, to be sure, but at the same time it is clear that 

 the trypanosome on its part does no harm. 



In other infections, as in anthrax particularly, harm is actually 

 done, but the symptoms of harm appear so late and are of such brief 

 duration that one is scarcely warranted in speaking of the existence 

 of an infectious disease; when symptoms arise death is virtually at 

 hand. In such infections as tetanus, diphtheria, and cholera, on the 

 other hand, symptoms of disease become very evident relatively 

 early after infection, and only too often appall us through their very 

 violence. 



On first consideration one might imagine that the severe symp- 

 toms in the one group and absence of symptoms in the other, are 

 merely the expression of a particularly active multiplication of the 

 organisms in the one as compared with the other, and of a cor- 

 respondingly severe intoxication of the macroorganism with toxic 

 metabolic products furnished by the invading parasite. 



This explanation, however, falls to the ground if we remember 

 that in the very group in which the most active and generalized 

 development of organisms occurs, symptoms of disease are virtually 

 absent, while in tetanus and diphtheria the infection is essentially 

 a local one, and the severity of the symptoms out of all proportion 

 to the small number of organisms present. There is, however, a 

 further important difference between the two groups of organisms. 



