INTRO.] MICRO-ORGANISMS AND DISEASE. 3 



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. 



B 2 



