HO RELATIONS OF BACTERIA TO DISEASE. 



will produce pathological changes if introduced in sufficient 

 quantity. When, therefore, we speak of a pathogenic 

 organism, the term is merely a relative one, and indicates 

 that in certain circumstances the organism will produce 

 disease, though in the science of human pathology it is 

 often used for convenience as implying that the organism 

 produces disease in man in natural conditions. 



Modifying Conditions. In studying the pathogenic 

 effects in any instance, both the micro-organisms and the 

 animal affected must be considered, and not only the species 

 of each, but also its exact condition at the time of infection. 

 In other words, the resulting disease is the product of the 

 sum total of the characters of the infecting agent, on the 

 one hand, and of the subject of infection, on the other. 

 We may, therefore, state some of the chief circumstances 

 which modify each of these two factors involved and, con- 

 sequently, the diseased condition produced. 



i. The Infecting Agent. In the case of a particular 

 species of bacterium its effect will depend chiefly upon (a) 

 its virulence, and (b) the number introduced into the body. 

 To these may be added (c) the path of infection. 



The virulence, i.e. the power of multiplying in the body 

 and producing disease, varies greatly in different conditions, 

 and the methods by which it can be diminished or in- 

 creased will be afterwards described (vide Chapter XIX.). 

 One important point is that when a bacterium has been 

 enabled to invade and multiply in the tissues of an animal, 

 its virulence for that species is often increased. This is 

 well seen in the case of certain bacteria which are normally 

 present on the skin or mucous surfaces. Thus it has been 

 repeatedly proved that the bacillus coli cultivated from a 

 septic peritonitis is much more virulent than when taken 

 from the bowel of the same animal. The virulence may 

 be still more increased by inoculating from one animal to 

 another in series the method of passage. Widely different 

 effects are, of course, produced on the virulence being 

 altered. For example, a streptococcus which produces 

 merely a local inflammation or suppuration, may produce a 



