Sir John Simon. 343 



Accordingly susceptibility depends on the presence of B, immunity 

 on its complete conversion. If this is admitted, it follows that the 

 human organism must be inhabited by as great an assortment of 

 specific susceptibilities as there are communicable diseases ; for, on the 

 one hand, the specificity of the contagium A necessarily carries with it 

 that of the substratum B, by the transmutation of which it is produced, 

 and, on the other, the immunity from a specific contagium which the 

 hypothesis accounts for as resulting from the transmutation, is a thing 

 quite as specific as the previous susceptibility. It follows from this 

 that the material of contagium must be a constituent of the blood not 

 essential to the performance of its nutritive functions (for otherwise the 

 acquirement of immunity would be fatal to life), the characteristic 

 facts relating to it being first, that it is convertible into contagium as 

 the immediate result of infection ; secondly, that as the consequence of 

 that transformation, it becomes liable to catalytic expulsion from the 

 organism ; and thirdly, that its elimination is equivalent to immunity. 



Whether or not Simon's reasoning can now be regarded as adequate, 

 it is of great interest to note that the fundamental notion of the normal 

 susceptibility of the organism was clearly enunciated by him 54 years 

 ago, as serving to explain the facts of infection as they then presented 

 themselves, and that now, after half a century of progress, the same 

 notion finds expression in terms more elaborate, but not essentially 

 different from those which were then employed. 



It was not until the seventies that Simon again wrote on the 

 subject. The great discoveries which, in the following decade, afforded 

 ground for the inference that each specific infection centres round a 

 specific concomitant micro-organism, had not yet been made, and 

 Simon was not destined to take part in them. To him the settlement 

 of the identity of each true contagium by the uniformity of its 

 operation as tested experimentally, was of greater interest than its 

 biological classification. He was well aware that in one or two instances 

 it had been proved experimentally that ' ' the specific microphytes of 

 disease " can be " conducted through a series of artificial cultivations," 

 and that " germs thus remotely descended from a first contagium will, 

 if living animals be inoculated with them, breed in these animals the 

 specific disease"*; and he anticipated that specific organic forms 

 might prove to be the " essential originators of infective processes " ; 

 but he conjectured that the progress of discovery would be slow. In 

 this he was mistaken. At the moment that he thus wrote, Koch had 

 already in hand the investigations which led to the discovery of the 

 bacteriological method, and in a few more years conjecture was 

 transformed into fact. 



"Public Health Reports," ed ; by Edward Seaton, M.D., London, 1887, p. 578. 



