CHAPTER IV. 

 INFECTION AND IMMUNITY. 

 IN the preceding chapters we have been dealing Patho- 



... genie and 



with the subject of bacteriology in the broadest sense, non-patno- 



J ** J . genie bac- 



Attention has been directed to the function of bacteria teria 

 in the life of the world, to their appearance, their 

 manner of growth, and the means employed for their 

 destruction. As physicians and nurses our interest 

 centers about a very small part of the bacterial king- 

 dom, the one having to do with the production of 

 disease. Bacteria that produce disease are termed 

 pathogenic, while those varieties that do not are 

 called non-pathogenic. By far the larger number of 

 pathogenic bacteria thrive only in the living tissues 

 of animals. These are called parasites. Some kinds 

 of bacteria thrive only on dead tissues or wounded 

 surfaces and, by decomposing them, form poisons 

 (ptomaines) which may be absorbed and give rise to 

 symptoms such as fever, chills, and headache. These infection 

 are termed saprophytes. When pathogenic bacteria 

 gain access to the tissues and produce injury and 

 symptoms, we say that infection has taken place. 



Here it may be well to say a word as to the mean- i nfec - 

 ing of the terms "infectious" and "contagious." They ti0 co S n - nd 

 have been used somewhat loosely and have led to a 

 great deal of confusion. Any disease that is caused 

 by the entrance into the body of a living micro- 



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