100 BACTERIOLOGY. 



elated with them. Again, it is found that the associa- 

 tion of one variety with another may increase its viru- 

 lence. Thus Roux and Yersiu believe that they have es- 

 tablished the fact that streptococci and diphtheria bacilli 

 mutually increase each other's virulence. On the other 

 hand, the absorption of the products of certain bacteria 

 immunizes the body against the invasion of other bac- 

 teria, as shown by Pasteur that attenuated chicken 

 cholera cultures produce immunity against anthrax. 



The Modes of Entrance of Infection. The various fluids 

 and tissues of the body differ greatly in their chemical 

 constituents, their reaction, their protection from in- 

 fection, their access to free oxygen, their temperature, 

 and in other less well-known respects. These varia- 

 tions are sufficient to render certain portions of the 

 body suitable for the growth of some bacteria and 

 unsuitable for others. This fact is of immense im- 

 portance in the transmission or prevention of disease. 

 Thus, for example, let us rub very virulent strepto- 

 cocci, typhoid bacilli, and diphtheria bacilli into an 

 abrasion on the hand. The typhoid bacillus produces 

 no lesion, the diphtheria bacillus but a very minute 

 infected area, but the streptococcus gives rise to a severe 

 cellulitis or fatal septicaemia. Now place the same 

 bacteria on an abrasion in the throat. The typhoid 

 bacillus is again harmless; the diphtheria bacillus pro- 

 duces inflammation, a pseudomembrane, and toxaemia, 

 and the streptococcus causes an exudate, an abscess, or 

 a septicaemia. Finally, introduce the same bacteria into 

 the intestines, and now it is the typhoid bacillus which 

 produces its characteristic lesions, while the strepto- 

 coccus and diphtheria bacillus are usually innocuous. 



If we tried in this way all the parasitic bacteria we 



