

VACCINE AND SERUM THERAPY 

 IN VETERINARY PRACTICE 



CHAPTER I 



IMMUNITY : ITS CAUSE AND ITS EFFECT ON 



DISEASE 



In contact with the bodies of animals there is a large 

 flora of micro-organisms, some as constant parasites, 

 others as transient invaders ; some harmless sapro- 

 phytes, and others capable of becoming pathogenic. 

 It is evident, therefore, that the production of infection 

 must depend upon other influences than the mere 

 presence of the micro-organisms and their contact 

 with the body, and that the occurrence of the reaction 

 — for the phenomena of infection are in truth re- 

 actions between the germ and the body defences — 

 is governed by a number of important secondary 

 factors. 



In order to cause infection it is necessary that the 



bacteria shall gain entrance to the body by a path 



adapted to their own respective cultural requirements, 



and shall be permitted to proliferate after gaining a 



foothold. Some of the bacteria then cause disease by 



rapid multiplication, progressively invading more and 



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