CHAPTER XII. 

 REACTION OF THE HOST TO INFECTION. 



Facts and Theories. The host reacts to the presence of a 

 pathogenic agent by a number of alterations in its physiological 

 activities. Some of these alterations are gross and well known 

 as the clinical manifestations of an infectious disease; others 

 require special search for their detection; while some, doubtless 

 a considerable number, still pass unobserved. Many of these 

 changes are susceptible of very accurate observation, and in 

 most instances the observed facts are well established. A clear 

 understanding of the relation of the various facts to each other 

 involves some imaginative reasoning, and various hypotheses 

 have been advanced to explain the phenomena observed, and to 

 fill in the gaps in our knowledge. The student may need to be 

 on his guard not to confuse facts susceptible of observation with 

 hypothetical deductions based upon such observations. Both 

 have their peculiar value. An understanding of the phenomena 

 of pathological physiology must be based upon correct ideas of 

 normal physiology and accurate knowledge has not fully replaced 

 hypothesis in this latter field. 



Physiological Hyperplasia. Under normal conditions each 

 cell of the human body is in close association with other cells 

 and with the body fluids, and is subject to the physical and 

 chemical stimulation of cells and fluids. One of the effects is 

 apparently to restrain the prolif erative activity of the cells. When 

 certain of these cells are destroyed, or even certain parts of them, 

 this restraint is removed, and the natural tendency to prolifera- 

 tion asserts itself, resulting in the production of new cells or of 

 new parts to replace the old, and usually more than compensates 

 for the loss. This somewhat hypothetical conception, due to 



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