166 THE RELATIONS OF MICRO-ORGANISMS TO DISEASE. 



generative, does not absolutely prove their etiological relationship to the 

 disease, although it renders it in a high degree probable. 



It is desirable in every case in which the evidence of the etiological 

 relationship of a specific micro-organism to a disease is to be set forth, 

 that we should be able to demonstrate the constant presence in the body 

 of the special form of micro-organism during some period of the disease ; 

 obtain this by culture in a pure condition unmixed with any other living 

 thing or with any chemical substance not belonging to it, and finally, by 

 the introduction of the purified organisms into a healthy animal, be able 

 to produce the disease in some definite form. When all this is done, and 

 not before, can we assert that the evidence establishing the causative 

 relationship between a given form of bacteria and any special infectious 

 disease, is entirely at our command. 



But the fulfilment of these strict logical requirements is very difficult 

 in many cases, and in some, apparently, almost if not quite impossible ; 

 for we must remember, in the first place, that the lower animals, upon 

 which alone, for the most part, inoculation experiments are practicable, 

 are apparently not subject to certain important diseases of man ; and, 

 second, that they present among themselves the most marked differ- 

 ences in the degree and manner in which they are affected by inocula- 

 tion with pathogenic bacteria. Desirable as is the complete fulfilment 

 of the above requirements in every case, it must be admitted that a rea- 

 sonable certainty regarding the bacterial origin of a given disease may 

 sometimes be arrived at without positive results from the inoculation of 

 the bacteria associated with its lesions. The agglutination test applied 

 under proper conditions may afford valuable evidence of the nature of 

 an infection (see p. 190). 



The complete demonstration which is desirable has as yet been fur- 

 nished in but a moderate number of diseases. In many others, however, 

 enough has been done in the way of study and experimentation to render 

 it altogether certain that they are infectious and to establish beyond 

 reasonable doubt the identity of the micro-organism or micro-organisms 

 involved. 



Conditions Influencing the Occurrence of Infectious 

 Diseases. 



It has been learned, as the result of a great deal of observation and 

 experiment, that although certain diseases are always associated with the 

 presence and growth in- the body of particular species of micro-organ- 

 isms, there are still various other accessory factors which have an im- 

 portant bearing upon the inception and course of the diseases. Thus, 

 while the presence in the body of a particular species of micro-organism 

 is the most significant and fundamental of the determining agencies in the 

 infectious diseases, the numbers in which they are present i.e., the size 

 of the dose and the varying virulence which the same species under 

 different conditions possesses, as well as the varying capacity of resist- 



