160 MICROBIOLOGY 



Channels of infection. Pathogenic bacteria gain access 

 to the body tissues through a variety of channels, the most 

 common of which are: (1) The digestive tract. In drinking 

 water, as the bacteria of water-borne diseases, and in milk. 

 Other foods frequently serve as vehicles by which infectious- 

 agents are carried into the body. Foods chemically changed 

 by the action of bacteria and made unfit for use and which 

 may cause poisoning are to be differentiated from infection. 

 The cause of the disturbance in these cases is either bacterial 

 products or cleavage products due to the splitting up of the 

 food substance into other compounds. (2) The respiratory 

 tract. Pathogenic bacteria may gain entrance through the 

 air passages. There are many who believe that tuberculosis 

 is most frequently contracted in this manner. People be- 

 come infected with anthrax (wool storers' disease) by in- 

 haling the spores of the anthrax organism. (3) The gen- 

 erative organs. This can happen when the disease is located 

 in the organs of reproduction and the virus is being elimi- 

 nated. The venereal diseases are illustrations of this means 

 of infection. Other diseases, such as tuberculosis, are 

 rarely transmitted through this channel. (4) Through wounds- 

 of the skin and mucous membrane. In addition to the com- 

 mon wound infections B. tetani usually gains entrance in this 

 way. Anthrax is transmitted in this manner, especially in 

 the form known as malignant pustule. Nearly if not all in- 

 fectious diseases can be transmitted through wounds. (5) 

 The bites of insects. This is practically the same as by 

 wounds. As insects play such an important part in the dis- 

 semination of many of the important diseases such as Texas 

 fever, the transmission by means of insects is very important 

 and liable not to be included 'with wound infections. 



Bacteria escape from the infected body. Pathogenic 

 bacteria are disseminated from the infected individual in 

 various ways. 



1. They escape through the excreta when the disease 

 is of such a nature that the bacteria are given off into the 

 channels of excretion. Tubercle bacteria appear in the feces. 



