CHAPTER VI. 



THE RELATION OF BACTERIA TO DISEASE- 

 IMMUNITY. 



THE difference between saprophytes and parasites has 

 already been emphasized and incidentally it has been learned 

 that the latter may for a short time lead a life comparable 

 to that of the former. The organisms that produce disease, 

 pathogenic, are everywhere, particularly in the crowded life 

 of cities. Not only are they on the objects of our environment, 

 but within the entrances to the body. Sometimes organisms 

 are found in the mouth and nose which are classed as patho- 

 genic. Certain organisms are present invariably in the 

 alimentary canal, which under proper circumstances can pro- 

 duce disease. It is often difficult, therefore, to determine 

 precisely how a bacterium enters the body and produces the 

 disease, because it is evident that some factors other than 

 the simple presence of microorganisms are necessary to 

 develop what is termed sickness. A disease might be fairly 

 well described as the subjective (experienced by the patient) 

 and objective (perceived by the physician) expression of the 

 forces exerted by the bacteria and the defence presented by 

 the body. 



These two forces must now be considered, and following 

 the natural sequence, bacteria will be traced in their usual 

 seats upon and within the human body, in their course past 

 the primary defences and their manner of awakening the 

 secondary or peculiar immunity resistances which the human 

 system presents. 



Bacteria gain entrance to the body by introduction through 

 an abraded surface of the skin or mucous membranes. The 



(59) 



