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tell in what this peculiarity consists. In other words, some 

 individuals are predisposed to the acute infectious fevers, 

 whilst others are not ; and as this predisposition cannot, so 

 far as I know, be acquired as the specific fevers are incap- 

 able of originating pathological habit by their repetition in 

 the same individual, and therefore are incompetent to 

 produce anything in the nature of a permanent diathetic 

 condition it must be inherited as a constitutional peculiarity. 

 We thus see that while the specific fevers, in their influence 

 upon those affected by them, leave no taint that may be 

 transmitted, and that so far as the majority of the exanthe- 

 mata are concerned, one severe attack of either of them 

 confers an immunity (which may be life-long) from secondary 

 attacks ; yet the fact remains that in almost every family 

 there are those who are either especially predisposed, or 

 more or less insusceptible to their influence, and that whilst 

 such predisposition consists in an unknown constitutional 

 peculiarity, it is only fair to assume that it has been produced 

 by heredity. Moreover, unless the insusceptibility has 

 resulted from the immunity bestowed by a former attack, 

 and has thus been acquired, it may also be regarded as a 

 constitutional peculiarity which has been inherited. 



I have said just now that one attack of an exanthem 

 may confer an immunity from secondary attacks which may 

 last during the life-time of the individual, but even this 

 immunity is often seen to be neither perfect nor universal ; 

 for there is not one of the exanthemata which may not 

 attack the same individual more than once, although a first 

 attack so modifies those which are secondary as to render 

 the symptoms, as a rule, much less severe, and comparatively 

 immature. In the case of vaccination, for example, it is a 

 well-known fact that while some persons lose its protective 



