PARASITISM AND PATHOGENESIS 195 



infestation with parasitic insects and numerous worms, in addi- 

 tion to an enormous variety of microbes. Whether a parasitk 

 organism is to be regarded as a symbiont, a commensal or a 

 pathogenic agent depends upon the effect which it produces 

 upon its host. A pathogenic organism is one whose presence 

 results in definite injury to the host. 



Pathogenesis. In human pathology the phenomena of dis- 

 ease have for centuries been the object of careful study and 

 speculation, and in many instances the phenomena commonly 

 associated together have long been regarded as a complex result 

 of a single primary cause, and the condition in which such phe- 

 nomena are observed has been regarded as a single morbid en- 

 tity or a definite disease. Even the most ancient records indicate 

 that such recognition had long been common knowledge. A 

 beginner in parasitology or pathology may be inclined to ascribe 

 a causal relation to a parasite which he observes in the body of a 

 sick individual; in fact this has been done repeatedly. The log- 

 ical requirements for the proof of such a relationship were first 

 formulated by Henle, as has been mentioned in the historical 

 sketch in the introductory chapter. They were reformulated 

 by Koch, who, for the first time, was able to comply with them 

 in respect to a bacterial disease. They may be stated as follows: 



1. The organisms must be present in all cases of the particular 

 disease. 



2. The organism must be isolated from the diseased body 

 and propagated in pure culture. 



3. The pure culture of the organism when introduced into 

 susceptible animals must produce the disease. 



4. In the disease thus produced, the organism must be found 

 distributed as in the natural disease. 



Although we may very properly consider a micro-organism as 

 the probable cause of a disease with which it is associated, with- 

 out satisfying all of the above requirements, the experience of 

 the last three decades has served to emphasize more and more 



