CONTAGIUM VIVUM. 31 



all to have this one characteristic : that in appropriate media 

 (among which must evidently be counted any living bodily texture 

 or fluid which they can infect) they show themselves capable of self- 

 multiplication, and it is in virtue of this property, that, although 

 at the moment of their entering the body they in general do not 

 attract notice either as objects of sense or as causes of bodily 

 change, they gradually get to be recognisable in both of these 

 respects. Now, the faculty of self-multiplication is eminently one of 

 the characters which we call vital ; and, when it is said that all 

 contagia are self-multiplying things, this is at least very strongly 

 to suggest that perhaps all contagia are things endowed with 

 life " (John Simon). 



Again, it has been pointed out that the distribution in place 

 and time of specific infectious diseases follows the same laws as the 

 distribution of organised beings. " Climiate and physical con- 

 ditions do not produce specific diseases. The origin of these is 

 as obscure as the origin of species ; their migrations are explicable 

 by the same laws as those of organised beings, and they often 

 accompany, like domestic animals and plants, the migrations of 

 man." (J. F. Payne.) 



It is only during the last ten or fifteen years that the existence 

 of micro-organisms as causes of disease has been definitely 

 proved^ and it cannot now be doubted that a contagium vivum is 

 an actually demonstrated reality. In the case of a good many 

 diseases the proof is still wanting, but analogy indicates that many 

 of the actual contagia, not yet discovered, will ere long be 

 definitely described. 



It is not a little remarkable that the two diseases, in which a 

 definite visible Bacillus has now been proved to be the one 

 essential factor in their production, should not be universally 

 admitted to be contagious. The bacillus of tubercle is now well- 

 known to be invariably present in tuberculous products, and the 

 bacillus of leprosy is an equally constant element in leprous 

 tissues ; and yet these two diseases are not commonly included 

 amongst the Enthetic class, whereas, in few of the ordinary 

 contagious Exanthemata has a definite living contagium been 

 discovered. No one doubts but that the list of demonstrable 

 pathogenic bacteria will gradually be added to, but we are already 



