CHAPTER IV. 



THE CAUSE OF INFECTIOUS DISEASE. 



THE wish " to know the cause of things " is 

 as old as mankind itself. In medicine the 

 scientific period may be said to dawn at the 

 moment when the question as to the connection 

 of disease with environment was clearly pro- 

 pounded by Diodorus and by Hippocrates, " the 

 father of medicine." 



In former times men were generally satisfied, 

 and they are frequently satisfied to-day, with 

 the vaguest conceptions of things,, conceptions 

 based on the common ground of a search after 

 animate causes or personifications. Universal 

 knowledge is plainly unattainable in any given 

 section of time ; hence men have always been 

 forced to piece out in imagination part of the 

 lacking facts, that is to theorize ; and the 

 form such speculation assumes is naturally in 

 accordance with the measure of cultivation 

 prevailing at the time. Even now, as is clearly 

 shown by the conceptions of image-worship 

 among both the educated and uneducated there 

 weighs upon wide strata of society an impera- 



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