CONDITIONS MODIFYING PATHOGENICITY. 123 



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, of micro-organisms varies greatly 

 in different conditions, and the methods by which it can 

 be diminished or increased will be afterwards described (vide 

 Chapter XIX.). Here it may simply be stated that widely 

 different effects may be produced by altering the virulence. 

 For example, a streptococcus which produces merely a local 

 inflammation or suppuration, may produce a rapidly fatal 

 septicaemia on its virulence being exalted. The virulence 

 also has a relation to the animal employed, as sometimes 

 on being increased for one species of animal it is diminished 

 for another. For example, streptococci, on being inocul- 

 ated in series through a number of mice, acquire increased 

 virulence for these animals, but become less virulent for 

 rabbits. (Knorr.) 



The number of the organisms introduced or the dose of 

 the infecting agent is another point of importance. The 



