METHODS AND CHANNELS OF INFECTION 669 



In leprosy also almost direct contact is necessary for a transfer of the 

 infectious agent. 



The Routes by Which Infectious Microorganisms Enter the 



Body 



Microorganisms enter the body through either the external or 

 internal surfaces. It has been shown that the absolutely healthy and 

 intact skin furnishes an efficient barrier to the entrance of infectious 

 agents. Pathogenic bacteria, for example, the streptococcus and the 

 various varieties of the staphylococci are present on the skin almost 

 continuously yet do not often produce infections. When there is an 

 abrasion of the skin or a diseased duct or hair follicle the bacteria 

 frequently pass down into the skin and an infection results. When 

 the pyogenic bacteria, such as mentioned above, are vigorously rubbed 

 into the skin infection sometimes takes place but in this instance also 

 there has been some mechanical injury to the skin. Minute unobserved 

 abrasions of the skin also serve commonly as points of entrance 

 for ,the Bad. pesHs of the plague. The microorganisms of tetanus, 

 anthrax, symptomatic anthrax and malignant edema always enter 

 the skin through wounds. Sometimes the infectious agents remain 

 local and at other times are carried from the point of the original 

 entrance. This may take place in different lengths of time. For 

 example, in tetanus the bacteria remain localized in most instances at 

 the point of the original wound and their toxin diffuses from this 

 point. In the various pyogenic infections the bacteria usually remain 

 locaUzed. However, in anthrax the bacteria are carried into the circu- 

 lation in a very few minutes after they enter the wound. In the new- 

 born infection very frequently enters the body through the umbilicus 

 or navel. Tetanus is one of the common diseases acquired in this way 

 in certain localities. Microorganisms may also enter the skin through 

 the wounds made by insects such as mosquitoes, flies, ticks, and fleas. 

 The larger and the clearer cut the wound the less the danger of infec- 

 tion because of the mechanical and bactericidal barrier of the fibrin and 

 the bactericidal action of the blood serum. A free flow of blood a,lso 

 washes the microorganisms out of the wound. Crushing wounds are 

 especially dangerous inasmuch as there is not a free flow of blood and 

 also there is a good chance for the growth of anaerobic bacteria such 



