THE DOCTRINE OF CONTAGIUM VIVUM. 329 



our very limited knowledge now extends, the pathophytes 

 hitherto discovered all belong to that group of the fungi which 

 are called bacteria. Now, fungi have two marked characteris- 

 tics, namely, the tendency to assume the parasitic habit, and 

 the possession by some of them of a special ferment action. 

 Both these characteristics may bear a part in the action of 

 pathogenic organisms. In the complex phenomena of sep- 

 ticaemia such would appear to be the case; a poisonous 

 ferment-product first intoxicates the system, and then the 

 organisms themselves prey upon the dead or moribund 

 tissues. 



There is, as Dr. B. Sanderson has pointed out, a marked 

 distinction to be drawn between those common processes of 

 infective inflammation which are shared in by animals 

 generally, such as septo-pyeemia, erysipelas, and the diph- 

 theritic process, and those specific contagia which are strictly 

 confined, like ordinary parasites, to particular species. 

 There is nothing in all nature more wonderful than the in- 

 timate and subtle nexus which unites a parasite to its host. 

 A hundred examples might be given. Even diff"erent varieties 

 or races of the same species have different and exclusive 

 parasites. It would seem as if this nexus depended on some 

 delicate shade, a nuance, something like an odour, or a savour, 

 or a colour, rather than on differences of structure or chemical 

 composition. The same minute correlation is seen in specific 

 contagia — all are strictly confined to one or a few species. 

 Yaccinia is confined to man, the horse, and the cow ; scarlet 

 fever is confined to man, and perhaps the swine ; most of 

 our specific diseases are absolutely confined to man. The 

 human and ovine smallpox, although so wonderfully similar, 

 are not intercommunicable. I am, therefore, inclined to 

 believe that, in regard to specific contagia, we shall find 

 more guiding analogies in parasitism than in fermentation. 

 Our information at present is, however, so defective that it 

 is not wise to enter into furthur speculations on this subject. 



Gentlemen, I have brought my task to a conclusion. I 

 believe that the doctrine of a contagium vivum is established 

 on a solid foundation; and that the principle it involves, if 

 firmly grasped in capable hands, will prove a powerful in- 

 strument of future discoveries. And let no man doubt that 

 such discoveries will lead to incalculable benefits to the 

 human race; our business in life is to do battle with dis- 

 ease, and we may rest assured that the more we know of 

 our enemy the more successfully we ?hall be able to comba^t 

 him. 



