ii MICBO-OKGANISMS AND DISEASE. [INTRO. 



to a particular micro-organism if any one of the following con- 

 ditions remains unfulfilled : (1) 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-organisms ; to go on cultivating them from one cul- 

 tivation 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 in- 

 stance. (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 dis- 

 ease, and in this way to show that this animal becomes affected 

 with the same disease as the one from which the organisms 

 were originally derived. (4) And, finally, it is necessary that 

 in this so affected new animal the same micro-organisms 

 should again be found. A particular micro-organism may 

 probably be the cause of a particular disease, but that really 

 and unmistakably it is so can only be inferred with certainty 

 when every one of these desiderata has been satisfied. 



It will be my aim in the following pages, first to describe 

 the methods that may be employed with success in investiga- 

 tions bearing on the relation of micro-organisms to disease ; 

 secondly, to describe in conformity with reliable observations 

 the morphology and physiology of the micro-organisms that 

 bear any relation to disease ; and thirdly, to enumerate the 

 observations that have been made in recent years to prove the 

 existence of such an intimate relation. Last, but not least, we 

 shall consider the precise relation of the particular micro- 

 organisms to the causation of disease. 



