94 INFECTION AND IMMUNITY. 



bacterial influence felt. Again, the different secretions of 

 the body have more or less germicidal effect, so that bacteria, 

 as a rule, are more potent in their effect when introduced 

 directly into the circulation. 



4. The association of bacteria among themselves has occa- 

 sionally the power of increasing the toxic effects of the inocu- 

 lated germs, sometimes the two germs acting simultaneously 

 on the animal body and producing what is known as " double," 

 "mixed," or "associated" infection. At other times, some of 

 the germs, though not pathogenic, are able to destroy the 

 resistance of the body to the action of other toxic germs, as, 

 for instance, the injection of tetanus bacillus with some 

 ordinary saprophyte is capable of producing symptoms when 

 the introduction of the tetanus germs alone would utterly 

 fail. 



Occasionally a beneficial association of germs may be ob- 

 served, the presence of the secretion of some bacteria being 

 prejudicial to the growth of other bacteria or neutralizing 

 their toxins. 



5. The condition of the human or animal body as to per- 

 fect health, as has already been remarked, offers more or less 

 resistance to the bacterial poison. When, however, the gen- 

 eral health is below par this resistance is diminished and the 

 animal is much more susceptible to the action of the germs. 



6. The time elapsed since the infection is often of great 

 moment. In some cases germs will lurk in an organ for a 

 long time, after which, through circumstances very little 

 understood, they will suddenly and violently begin to cause 

 symptoms and often death. Diseases of the appendix and 

 gall-bladder in man are among the more familiar examples 

 of this phenomenon. 



IMMUNITY AND ITS VARIETIES. 



Resistance to the action of pathogenic bacteria is called 

 immunity, and is either natural or acquired. 



I. Natural immunity is present in all such cases where, for 

 instance, some species of animals can not be affected by cer- 



