DIVISION VII. 

 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



CHAPTER I.* 

 METHODS AND CHANNELS OF INFECTION. 



INFECTION DEFINED. 



Infection of man or animals implies the entrance of pathogenic organ- 

 isms, their multiplication and their capacity to do injury. In most 

 instances the organisms enter the tissues of the body, although this is 

 not true in every case. It is possible to produce the symptoms of infec- 

 tion by the use of the chemical products elaborated by pathogenic 

 organisms when these products are injected. As an example may be 

 mentioned the injection into the body of the chemical products of B. 

 tetani; there results therefrom the typical symptoms of tetanus. How- 

 ever, these products do not occur naturally without the pathogenic or- 

 ganisms, and therefore they do not produce infections in the usual sense. 



The disease producing microorganisms with which we shall especially 

 concern ourselves in the following discussion are of three kinds: first, 

 bacteria; second, protozoa, and third, ultramicroscopic microorganisms. 



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

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

 sion. An infectious disease is any disease which is due to a micro- 

 organism. The name is applied to the nature of the cause of the disease. 

 A contagious disease is an infectious disease which is transmitted 

 from one individual to another by direct contact or by the agency of 

 fomites. It has to do with the method of transmission rather than the 

 cause of the disease. 



* Prepared by E. F. McCampbell. 



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