2 ETIOLOGY 



the fact that its location in the body of the diseased animal 

 is known, that with the morbid tissues the disease can be 

 produced in healthy animals and that without this definite 

 infection, no matter what the conditions are, the malady can- 

 not be made to develop, argues against extraneous conditions 

 as exciting causes. 



The mystery which formerly surrounded the origin, the 

 course and the disappearance of animal plagues has in a large 

 degree been cleared away ; and in its place we are confronted 

 with the problems involved in the life history and the possi- 

 bilities of invading microorganisms. In fact during the last 

 few years the biological sciences have been brought into im- 

 mediate use by the pathologists. Etiology has become per- 

 manently linked to microbiology so that in seeking for the 

 specific cause of an infectious disease we look for some species 

 of organic life which may belong either to the animal or to the 

 vegetable kingdom. The fact that certain microscopic animals 

 and plants have become, if they were not in the beginning, 

 parasitic on larger and higher forms of life has long been 

 recognized ; but the idea came later, that the various infections 

 giving rise to a wide series of phenomena, known as symp- 

 toms and morbid anatomy, were actually and simply the results 

 of the invasion of the individual with living microscopic plants 

 (bacteria) or animals (protozoa). It is likewise true that for 

 •many of the disorders consisting of changes recognized in the 

 terms of general pathology, the cause may be found in the 

 conditions of life under which the individual has been forced 

 to exist. Etiology, therefore, in a broad sense, includes both 

 the infecting microorganisms that produce the specific infectious 

 diseases and poor hygienic, unsanitary conditions and physical 

 forces which may produce non-specific morbid changes often 

 sufficient to cause death. 



§ 2. Infection. The term infection has come to be gen- 

 erally understood to mean the entrance into the animal body, 

 from without, of living microorganisms capable of multiplying 

 within the living tissues and of producing in consequence of 

 this multiplication a local or a general diseased condition and 



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