2 MICRO-ORGANISMS AND DISEASE. [INTRO. 



founded and experimentally proved facts, the statements set 

 forth in a particular observation or series of observations are 

 not to be accepted. 



In all investigations of the relation of micro-organisms to 

 disease it is necessary to bear in mind that, as Koch 1 has 

 pointed out, no observation can be said to be complete, or, 

 one should rather say, in no instance can it be said to have 

 been satisfactorily proved, that a particular infectious disease 

 is due to a particular micro-organism if any one of the 

 following conditions remains unfulfilled : (i) It is absolutely 

 necessary that the micro-organi-sm in question is present 

 either in the blood or the diseased tissues of man or of an 

 animal suffering or dead from the disease. In this respect 

 great differences exist, for in some infectious diseases the 

 micro-organisms, although present in the diseased tissues, are 

 not present in the blood ; while in others they are present 

 in large numbers in the blood only or in the lymphatics only. 

 These points will be considered hereafter in the special 

 cases. (2) It is necessary to take these micro-organisms 

 from their nidus, from the blood or the tissues as the case 

 may be, to cultivate them artificially in suitable media, i.e. 

 outside the animal body, but by such methods as to exclude 

 the accidental introduction into these media of other micro- 

 organisms ; to go on cultivating them from one cultivation 

 to another for several successive generations, in order to 

 obtain them free of every kind of matter derived from the 

 animal body from which they have been taken in the first 

 instance. (3) After having thus cultivated the micro- 

 organisms for several successive generations it is necessary 

 to re- introduce them into the body of a healthy animal 

 susceptible to the disease, and in this way to show that this 

 animal becomes affected with the same disease as the one 

 1 Die Mihbrand-impfung, Cassel and Berlin, 1883. 



