CHAPTER VI. 



THE RELATION OF BACTERIA TO DISEASE 

 -IMMUNITY. 



THE difference between saprophytes and parasites 

 has already been emphasized, and incidently 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 

 pathogenic. Certain organisms are present invariably in 

 the alimentary canal, and under proper circumstances 

 can produce 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 de- 

 scribed as the subjective (experienced by the patient) 

 and objective (perceived by the physician) expression 

 of the forces exerted by the bacteria and the defense 

 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, 



