THE PHENOMENA OF INFECTION 83 



which I have developed in my studies on the 

 chemistry and toxicology of bacterial and other 

 proteins. Only a living thing can infect. In- 

 jection of diphtheria or tetanus toxin may 

 cause all the symptoms and lesions of the re- 

 spective diseases, but such injections are arti- 

 ficial procedures and the results are intoxica- 

 tions rather than infections. In this section I 

 shall omit diseases due to toxins. The infect- 

 ing agent is a virus and in infections there is a 

 contest between the invader and the native. It 

 is a struggle for food, growth, and reproduc- 

 tion. In the bacterial diseases the structure or 

 the equipment of the invader is quite as compli- 

 cated and as complete as that of the defender. 

 The contest is between bacterial and body cells 

 and the battlefield may involve only a small 

 part or may extend to every part of the ani- 

 mal's body. 



What is the difference between pathogenic 

 and nonpathogenic bacteria? In order for a 

 given bacterium to be pathogenic to a given 

 animal it must be possible for the former to 

 feed upon the latter. All living things feed by 

 means of digestive ferments. Continued life and 

 multiplication are impossible under other con- 

 ditions. First, in order for a given bacterium 

 to infect a given animal the ferments, of the 



