PORTALS OF ENTRY OF INFECTIOUS AGENTS. 167 



quence of the peculiarities just explained, a microbe 

 cannot do harm if it enters the body unless it finds 

 lodgment in a situation favorable to its growth. In 

 other words, the microbe and opportunity must meet 

 for the production of disease. Example: The tetanus 

 (lock-jaw) bacillus is fatal if it enters a wound however 

 trivial, yet it may be swallowed (indeed, often is) 

 with impunity. 



The portals of entry of infectious agents and parasites 

 into the body may be included under the following 

 heads : 



1. Through wounds of skin and mucous membranes. 



2. By the mouth or nose, through air. 



3. By mouth, through food and water. 



4. By genito-urinary tract. 



5. By placenta (congenital infection). 



6. Cryptogenic (unknown). 



The immediately accessible portions of the body to 

 infectious agents are either covered with skin or mucous 

 membranes, so that in the production of disease, one 

 of those surfaces must be penetrated.* 



If an infection can be contracted either through 

 wounds of the skin or mucous membranes, it is said to 

 be inoculable. It is still a question in dispute whether 



*To make this generalization clear it is only necessary to remind the 

 reader that the air-passages have a mucous Hning from their external 

 openings down to their final terminations in air cells in the lungs; and 

 that the alimentary tract is lined with an uninterrupted sheet of mucous 

 membrane from inlet to outlet. 



