MECHANISM OF INFECTION AND IMMUNITY 227 



and the fixed cells elaborate destructive weapons. Which of these 

 bears the brunt of the defense depends on the armament of the invader. 

 Whether a given bacterium is pathogenic to a given animal or 

 not, depends essentially on two things. First, can it feed on the pro- 

 teins of that animal's body ? If it cannot, it can do no harm. Secondly, 

 can the cells of the body destroy the invading cells before they can 

 multiply ? 



The Phenomena of Infection. It should be clearly understood 

 that only a living thing can infect. It must not only be alive, but it 

 must be able to multiply in the animal body. It is true that the injec- 

 tion of diphtheria or tetanus toxin into an animal may cause all the 

 symptoms and lesions of disease, but this is an artificial procedure, and, 

 besides, the toxin is the product of bacterial growth. An infectious 

 disease arises when foreign cells find their way into the body and 

 multiply to the detriment of the body cells. Simply carrying virulent 

 bacteria on the surface or in the cavities of the body does not constitute 

 infection. It is not rare to find tubercle bacilli on the hands of those 

 who care for others ill of this disease. According to Fliigge, 70 per 

 cent, of those in houses where there is a case of epidemic meningitis 

 carry the organisms. In a schoolroom in which a child has developed 

 diphtheria, 30 per cent, of all the children may have the diphtheria 

 bacillus in their throats and are not infected. In order to develop 

 infection, the bacterium must feed on the body. Carriers of infection 

 are of importance to the epidemiologist, but they are not necessarily 

 infected. The bacterium must not only feed on the animal tissue, but 

 it must multiply. The essential difference between saprophytic and 

 pathogenic bacteria is that the latter can multiply in the animal body, 

 while the former cannot. Saprophytic bacteria contain in their cellular 

 substance just as much protein poison as the pathogenic organisms 

 do and it is easy to kill an animal by injecting a relatively large amount 

 of them into the abdominal cavity, but this is not infection. A bacterium 

 is not pathogenic to a given animal unless it can convert that animal's 

 proteins into its own proteins. 



Saprophytic bacteria are speedily digested by the enzymes in the 

 blood and tissues of the body, and if they be injected in large amount 

 the protein poison set free may be sufficient to quickly kill the animal. 



