METHODS AND CHANNELS OF INFECTION 675 



a very severe type. The results of the infection will be materially 

 modified depending on the avenue of entrance which the virulent micro- 

 organism takes. For example, in addition to those mentioned previ- 

 ously, suppose Bad. pestis, the causal agent of plague, enters the 

 blood through the skin, or the lymphatics through the tonsils, it is 

 carried to the lungs and there produces a very severe and usually fatal 

 pneumonia; if bacteria enter the lymphatic system in large numbers 

 they frequently localize in the lymph glands producing buboes or 

 glandular enlargements which are not always fatal. These bacteria 

 may also enter the blood current and produce a rapidly fatal septicemia. 

 Tt has not been established in man that plague can be produced by the 

 ingestion of Bad. pestis, but in some susceptible animals such as rats, 

 the disease in a very fatal form is rapidly acquired when the bacteria 

 enter the intestines. 



RESISTANCE. This factor is one of the prominent ones which 

 modify the results of an infection. It is a familiar fact that two or 

 more individuals may be infected with the same microorganism, as 

 for example, B. typhosus, and one will not become infected or have a 

 very mild form of the disease, while the other will have the severest and 

 most fatal form of typhoid fever known. Again, the age of the indi- 

 vidual infected is important in determining the resistance. The adult 

 resists infection such as diphtheria, scarlet fever, and measles more 

 than the child. The very young child resists pneumonia and tuber- 

 culosis more than the adult. The resistance of the body depends on 

 the presence in that body of natural or acquired antibodies. It is, 

 therefore, obvious that the higher resistance or immunity of the indi- 

 vidual infected, the less severe will be the results of the infection on 

 that individual. 



* 



THE EXACT CAUSE OF INFECTIONS 



We are familiar with the fact that all of our infectious diseases are 

 due to microorganisms or viruses of some form or other. The causal 

 agents of many of these diseases are known but in the case of those that 

 are not known there is reasonable certainty as to the types of the in- 

 fecting agents. The exact substances which are produced by the micro- 

 organisms and which are responsible for symptoms of the various 

 diseases will be briefly considered in the following paragraphs. 



