2 INFECTIOUS AND^PAfeASITIC DISEASES. 



the harmony of health, in other words, whatever causes 

 disease, always acts so as to destroy some tissue, or to 

 interfere with the functions of some organ. The mod- 

 ern conception of the causation of disease no longer 

 gives credence to the belief that its causes are novel 

 and mysterious, but, on the contrary, from indisputable 

 evidence, places them among the actual phenomena of 

 the physical world. Nor does it admit the spontaneous 

 development of disease in either the born or the unborn, 

 albeit obvious instances to the contrary, since in such 

 cases some external agency has previously acted directly 

 or indirectly (e.g., through a parent) as a first cause, 

 and interrupted the orderly continuity of the normal 

 functions. Therefore, no matter what the exciting 

 cause of a disease may be, it invariably, and primarily , 

 comes jrom without the body. 



When an extraneous cause acts injuriously upon a 

 person, his organism does not remain passive, but 

 reacts with all its might to counteract the cause and 

 repair any defect the latter' s presence has given rise to. 

 This power of the body to protect itself lays in a defen- 

 sive mechanism evolved during countless ages of suc- 

 cessful efforts to survive destruction against harmful 

 influences in the universal struggle for life. So nicely 

 is the body's mechanism of defense adjusted, that con- 

 stant re-adjustments are made to passing dangers, 

 under ordinary circumstances, without consciousness 

 being disturbed. In this way equilibrium with the 



