DIVISION VIII 

 Microbiology of Diseases of Man and Domestic Animals 



CHAPTER I* 

 METHODS AND CHANNELS OF INFECTION 



Infection Defined 



The term infection implies the entrance of animal or vegetable 

 organisms into the body of another animal or plant, their multiplica- 

 tion and their injury to that body. In most instances the organisms 

 enter the tissues of the animal or plant body, although this is not true 

 in every case of infection. It is possible in certain instances to produce 

 the symptoms of an infection by introducing into the body the 

 chemical products elaborated by some pathogenic organisms. For 

 example, the injection of tetanus toxin into the body causes the typical 

 symptoms of tetanus to result. Tetanus toxin is made by growing 

 B. tetani in beef broth under anaerobic conditions and filtering out the 

 bacteria by passing through porcelain filters. These chemical prod- 

 ucts do not occur naturally unassociated with the pathogenic 

 organisms and therefore they do not produce infections when artifi- 

 cially injected in the usual sense. 



The disease-producing organisms with which we will especially 

 concern ourselves in the subsequent discussion are those which are very 

 minute in size and are of three kipds: first, bacteria; second, protozoa; 

 and third, ultramicroscopic microorganisms or viruses. 



It is essential to have clearly in mind what is meant by an infectious 

 disease and a contagious disease before entering into any detailed dis- 

 cussion, although some authorities attempt to make no distinction. 

 An infectious disease is any disease produced in the plant or 

 animal body which is due to a foreign animal or plant organism. The 

 name is applied to the nature of the cause of the disease. A contagious 



• Prepared by E. F. McCampbell. 



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