22 INFECTION AND IMMUNITY. 



CHAPTER II. 



INFECTIOUS ETIOLOGY. 



It is evident that the discovery of the specific 

 organism of an infectious disease is of the great- 

 est importance for purposes of serum therapy, vac- 

 cination and hygienic prevention. For the demon- 

 stration of a virus, it is not in all cases necessary, 

 though desirable, that the organism be cultivated 

 artificially, nor that it be recognized visually. The 

 conditions in rinderpest may be cited in which the 

 body fluids of a diseased animal, known to contain 

 the infectious agent, are used for immunization, 

 although the microbe itself can not as yet be culti- 

 vated or recognized. 



Laws. There are so many possibilities of error, and so 

 many errors have actually been made in regard to 

 infectious etiology, that certain requirements in 

 the way of proof are now habitually demanded be- 

 fore a particular organism can be accepted as the 

 cause of a disease. These requirements are most 

 frequently expressed in the form of Koch's laws, 

 which may be stated as follows 1. The suspected 

 organism must be found constantly in the proper 

 tissues of an animal suffering from the disease, or 

 which has died from it. 2. The organism must be 

 cultivated artificially in a pure state. 3. It must 

 be possible to reproduce the disease in a suitable 

 animal by inoculation with the pure culture. 4. 

 The organism must again be cultivated in a pure 

 state from the tissues of the experiment animal. 



