CHANNELS OF INFECTION 5 



brought with the food or water. Hog cholera infection is a 

 good ilhistration of this mode of entrance. 



(2 ) Through the respiralorv trad. Bacteria are taken into 

 the lungs where they are brought with the inhaled atmosphere. 

 Pulmonary tubercular affection is often brought about in this 

 wa}-. 



( 3 I Through abrasions of the skin. The wide distribu- 

 tion of bacteria in nature renders it highly probable that in all 

 wounds of the integument microorganisms will reach the fresh 

 tissues. The}- may come from the cutting or tearing implement, 

 the particles of dirt which may fall into or upon the cut .sur- 

 face, or from the ducts of the glands of the skin itself. It may 

 happen that the fresh tissues thus exposed are infected with 

 one, or, with several .species of bacteria. It may be that one 

 or more of these species ma>- be destroyed Ijy the living juices 

 of the body or by the leucoc_\ tes, or, again it is possible that, 

 from their saprophytic nature, they may not be able to mul- 

 tiplv in this new environment ; in either ca.se the infection is of 

 no significance and clinically would not be recognized. It may 

 happen that only one species of the infecting bacteria multiplies 

 and produces the morbid changes. This would be a single in- 

 fection. If, however, two or more species co-operate in the pro- 

 duction of the lesions, it is called a mixed infection. This term 

 is often used to designate the condition where one species may 

 be responsible for the tissue changes although other bacteria 

 are present but in an accidental or passive way ( S 4J- 



(4; Through the generative organs. Infection of the re- 

 productive organs takes place in certain in.stances where they 

 are the seat of the disease. This is especially true in case of 

 maladic du coit. 



fsj Through the agency of insects. Some insects carry 

 the virus of certain diseases from the infected and introduce it 

 into the susceptible individuals. Thus the mosquito carries 

 the Plasmodium of human malaria, the cattle tick the piroplasma 

 of Texas cattle fever and f5ies are often the introducers of 

 pathogenic bacteria, such as those of anthrax (§ 76). 



( 6 ) Transmission of the virus from the parent to thejoetus. 

 Occasionally the young of diseased parents are born infected 



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