CHAPTER VI 



VARIATIONS IN VIKtJLENCB 



The pathologist is apt to forget that the vast majority of 

 bacteria are non-pathogenic, that is to say, they are harmless 

 to man. Not only so, but the activities of many of them are 

 as beneficial to him as those of the pathogenic bacteria are 

 the reverse. The purification of sewage and the mineraliza- 

 tion of dead vegetable matter, to mention only two instances 

 of bacterial action, are processes which contribute to the 

 health and survival of the human race no less than the 

 processes of disease conduce to its decay. 



The power to cause disease depends upon two factors. In 

 the first place, it depends upon the ability of the organisms 

 to become parasitic, that is to say, to invade the living 

 tissues and live and multiply there, and, in the second place, 

 it depends upon the result of their activity, more especially 

 as regards the formation of poisons or toxins. 



The harmless nature of most bacteria is due to the fact 

 that they have not acquired the power of becoming parasitic. 

 In some instances, where bacteria do succeed in invading 

 the tissues, the result of their activity within the body is 

 apparently harmless. Ford (1900) examined the liver and 

 kidneys of healthy animals after death, with the most stringent 

 precautions against contamination, and found at least 80 per 

 cent, contained bacteria of various kinds. In other cases, 

 though organisms fail to invade the living tissues, they 

 nevertheless produce symptoms of disease by manufacturing 

 toxins which are absorbed, as, for example, in puerperal 

 sapraemia. 



The results produced by the presence of bacteria in the 

 tissues are due to a variety of causes, which include (a) the 

 metabolism of the living organisms, that is to say, the 



