100 BACTERIOLOGY. 
ciated with them. Again, it is found that the associa- 
tion of one variety with another may increase its viru- 
lence. Thus Roux and Yersin 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 septicemia. 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 toxemia, 
and the streptococcus causes an exudate, an abscess, or 
a septicemia. 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 
