150 RELATIONS OF BACTERIA TO DISEASE 



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, 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 (6) 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 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 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 

 septicaemia when its virulence is raised. Virulence also has a 

 relation to the animal employed, as occasionally on being in- 

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

 For example, streptococci, on being inoculated in series through 

 a number of mice, acquire increased virulence for these animals, 

 but become less virulent for rabbits. (Knorr.) 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, and it is only in a few instances that one or 

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

 case of anthrax in white mice. The healthy peritoneum of a 

 rabbit can resist and destroy a considerable number of pyogenic 

 micrococci without any serious result, but if a larger dose be 



