INTRO.] MICRO-ORGANISMS AND DISEASE 3 



more the worker in this field, should be enabled to criticise 

 the observations and facts brought forward by the numerous 

 writers- on this subject, for otherwise he would probably 

 take as proved what has really not passed beyond the stage 

 of possibility. And it is this point which requires the most 

 careful attention — viz., to be able to see at a glance that, 

 owing to the imperfect or faulty methods employed, or that, 

 owing to certain inferences incompatible with the general 

 laws and general tendency of the well-founded and ex- 

 perimentally 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^ 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 morbid process 

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

 following conditions remains unfulfilled : — (i) It is absolutely 

 necessary that the micro-organism 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- 



' Die Mthbrand-iinpfung, Cassel and Berlin, 1883. 



E 2 



