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 respiratory tract. Bacteria are taken into 

 the hmgs where they are brought with the inhaled atmosphere. 

 Pulmonary tubercular affection is often brought about in this 

 way. 



(3) Throu^li abrasions of the skin. The wide distribu- 

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

 wounds of the integument microorganisms will reach the fresh 

 tissues. They maj- 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 may be destroyed by the living juices 

 of the body or by the leucocytes, or, again it is possible that, 

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

 tiph^ in this new environment ; in either case 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 (§ 4). 



(4) Through the generative organs. Infection of there- 

 productive organs takes place in certain instances where they 

 are the seat of the disease. This is especialh' true in case of 

 nialadie du coit. 



(5) T/irough the agejier 0/ 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 flies are often the introducers of 

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



(6) Transmission 0/ the virus from the parent to thefoettis. 

 Occasionally the young of diseased parents are born infected 



