2 ETIOLOGY 



cause is, the fact that its location in the body of the diseased 

 animal is known, that with these morbid tissues the disease can 

 be produced in healthy animals and that without these definite 

 infections, no matter what the surroundings are, they cannot 

 be made to develop, argues against extraneous conditions as 

 exciting causes. 



The mysteries which formerly surrounded the origin, the 

 course and the disappearance of animal plagues have in a large 

 degree been cleared awa}- and in their place we are con- 

 fronted with the problems involved in the life history and the 

 possibilities of invading microorganisms. In fact during the 

 last few years the biological sciences have been brought into 

 immediate use by the pathologist. Etiology has become per- 

 manently linked to microbiology so that in seeking for the 

 specific cause of an infectious disease we look for some spe- 

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

 to the vegetable kingdom. The fact that certain animals and 

 plants have become, if the}' were not in the beginning, para- 

 sitic on other 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 symptoms 

 and morbid anatomy, were actually and simply the results of 

 the invasion of the individual with living microscopic plants 

 (bacteria) or animals (protozoa J. 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 con- 

 ditions of life under which the individual has been forced to 

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

 infecting or parasitic microorganisms which cause infections 

 and produce the specific infectious diseases and poor hygienic, 

 unsanitar}' conditions and physical forces which maj- 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 

 thereof a local or a general diseased condition or the death of the 

 individual. The invading microorganisms may belong to any 



