31 a DISEASES Sect. XXXIK. 2. 9. 



the infected perfon ; as is evident from many perfons having 

 been near patients of the fmall-pox without acquiring the 

 difeafe. 



The reafon.whv many of thefe difeafes are received but once, 

 a-nd others repeatedly, is not well understood ; it appears to me, 

 that the constitution becomes fo accuitomed to the Stimuli of 

 thefe infectious materials, by having once experienced them, 

 that though irritative motions, as hectic fevers, may again be 

 produced by them, yet no fen Cation, and in confequence no 

 general inflammation fucceeds ; as difagreeable fmells or tafr.es 

 f>y habit ceafe to be perceived •, they continue indeed to excite 

 irritative ideas on the organs of fenfe, but thefe are not Succeed- 

 ed by feniation. 



There are many irritative motions, which were at firSt fuc- 

 ceeded by fenfation, but which by frequent repetition ceafe to ex- 

 cite fenfation, as explained in Seel:. XX. on Vertigo. And, that 

 this c ircum (lance exifts in refpect to infectious matter appears 

 from a k:io\vn fact \ that nurfes, who have had the fmall-pox r 

 are liable to experience fmall ulcers on their arms by the contact 

 of variolous matter in lifting their patients; and that when pa- 

 tterns, who have formeily had the fmall-pox have been inocula- 

 ted in the arm, a phlegmon, or inflamed fore, has fucceeded, but 

 no fubfequent fever. Which (hews, that the contagious matter 

 of the fmall-pox has not loft its power of Stimulating the part it. 

 feapplied to, but that the general fvftem is not affected in con- 

 fequence. See Section XII. 7.6. XIX. 10. 



o. From the accounts of the plague, virulent catarrh, and 

 putrid dyiVntery, it feems uncertain, whether thefe difeafes are 

 experienced more than once ; but the venereal difeafe and itch 

 arc doublets repeatedly infectious ; and as thefe difeafes ar * 

 never cured fpontaneoufly, but require medicines, which act 



ithout apparent operation, feme have fufpected, that the con- 



gious material produces fimilar matter rather by a chemical 



ei of the fluids, than by ah animal procefs; and that the fpecif- 



edicines deftroy their virus by chemically combining with it. 



Ti inion is fuecefsfiilly combated by Mr. Hunter, in his 



Treat ' 1 Venereal Difeafe, Part I. c. i. 



But riii-s opinion wants the fupport of analogy, as there is no 

 fen in animal bodies, which is purely chemical, not 



trend ion ; nor c;m any of thefe matters be produced by 



Bfoical pi >cefl I to this, that it is probable, that the 



infectr>. ot in the puftuies of the itch, .ind in the ftoolsof 



<Jyfenteric pa he conferences, and not the caufes of 



thtfedi • And that the fpecific medicines, which cure the 



bj at 1 as brimftone and mercury, act only by in- 



creasing 



