CONDITIONS MODIFYING PATHOGENICITY 175 



two factors involved, and, consequently, the diseased condition 

 produced. 



1. 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 increased will be 

 afterwards described {vide Chapter XXII.). 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 that 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 rapidly 

 fatal septicemia when its virulence is raised. Virulence also 

 has a relation to the animal employed, as occasionally on being 

 increased for one species of animal it is diminished for another. 

 For exa'mple ^streptococci. on being inoculated in seriesjhrough 

 aTmmberoT mice, acquire increJSed _ vTrulence~foFthese animals, 

 but~~become Te^r~vrru1en rTor rab bits (Knorr)f - Certain'facts 

 suggMF'fhaTT^enTmay be a periodicity in virulence, i.e., that an 

 organism may for a time produce a relatively mild type of 

 disease and then develop into a more potent strain capable of 

 overcoming the resistance of a greater number of individuals ; this 

 would account for the fact that in some diseases widespread 

 epidemics occur at almost fixed intervals of years. The 

 theoretical consideration of virulence must be reserved for a 

 later chapter (see Immunity). 



- The number of the organisms introduced, i.e., the dose of the 

 infecting agent, is another point of importance. The healthy 

 tissues can usually resist a certain number of pathogenic 

 organisms of given virulence, a good example being often fur- 

 nished in ulcerative endocarditis in man, where bacteria may 

 circulate in the blood for a considerable time without causing 

 secondary lesions. It is only in a few instances that one or two 

 organisms introduced will produce a fatal disease, e.g., the case 



