INFECTION AND INTOXICATION. 69 



bacilli respectively, it should never be forgotten that 

 under natural conditions infection of an unmixed charac- 

 ter is made practically impossible by the prevalence of 

 bacteria in general ; consequently, the streptococcus can 

 scarcely enter the skin without being accompanied by the 

 ever-present staphylococci ; the diphtheria bacillus finds 

 bacteria already in the throat in which it begins to grow ; 

 and the soil with which the tetanus bacillus enters the 

 body carries with it quite a flora of its own. It is true 

 that these accessory bacteria are in general unimportant ; 

 yet that this is not entirely true is shown by the fact that 

 the streptococcus which frequently grows in company 

 with the diphtheria bacillus in the throat greatly modi- 

 fies the course of the disease. Furthermore, it is the 

 streptococcus that is usually responsible for the meta- 

 static abscesses that develop. Again, because of its strictly 

 anaerobic nature the tetanus bacillus may be entirely 

 unable to grow in the tissues without the presence of 

 aerobic bacteria to withdraw the oxygen from it. 



Roget found the combination of the bacillus of malig- 

 nant edema and Bacillus prodigiosus greatly more viru- 

 lent than the malignant edema bacillus by itself. Giarre 

 found combinations of pneumococci and diphtheria ba- 

 cilli caused a great increase in the effect of the pneumo- 

 coccus. Ghadialli found a micrococcus which destroyed 

 both the typhoid and colon bacilli. 



It is interesting to observe that the bacteria secured 

 from cases of disease during epidemics are usually more 

 virulent than those secured from sporadic cases, probably 

 because of a natural selection by which the micro-organ- 

 isms passing from patient to patient undergo an exaltation 

 of virulence very similar to that carried out experimen- 

 tally in the laboratory. 



2. Number plays an important role in infection. It 

 is true that in a few diseases (anthrax), when the viru- 

 lence of the bacterium and the susceptibility of the ani- 

 mal (guinea-pig) are alike great, the introduction of a 

 single bacterium may be fatal. 



