3 iz DISEASES. SECT. XXXIII. 2.9, 



the infefted perfon ; as is evident from many perfons having 

 been near patients of the fmall-pox without acquiring the 

 difeafe. 



The reafon, why many of thefe difeafes are received but once, 

 and others repeatedly, is not well underftood ; it appears to me 

 that the conilitution becomes fo accuftomed to the ftimuli of 

 thefe infeftious materials, by having once experienced them, 

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

 produced by them, yet no fenfation, and in confequence no 

 general inflammation fucceeds , as difagreeable fmells or taftes 

 by habit ceafe to be perceived ; they continue indeed to excite 

 irritative ideas on the organs of fenfe, but thefe are not fucceed* 

 ed by fenfation. 



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

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

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

 this circumftance exifts in refpett to infectious matter appears 

 from a known fact , that nurfes, who have had the fmail-pox 

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

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

 tients, who have formerly 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 itimulatinglthe part it 

 is applied to, but that the general fyftem is not affected in con- 

 fequence. See Section XIL 7. 6. XIX. 10. 



9, From the accounts of the plague, virulent catarrh, and 

 putrid dyfentery, it feems uncertain, whether thefe difeafes are 

 experienced more than once ; but the venereal difeafe and itch 

 are doubtlefs repeatedly infectious ; and as thefe difeafes are 

 never cured fpontaneoufly, but require medicines, which act 

 without apparent operation, fome have fufpected, that the con- 

 tagious material produces fimilar matter rather by a chemical 

 change of the fluids, than by an animal procefs ; and that the fpe- 

 cific medicines deftroy their virus by chemically combining with 

 it. This opinion is fuccefsfully combatted by Mr. Hunter, in 

 his Treatife on Venereal Difeafe, Part I. c. i. 



But this opinion wants the fupport of analogy, as there is no 

 known procefs in animal bodies, which is purely chemical, not 

 even digeftion ; nor can any of thefe matters be produced by 

 chemical procefles. Add to this, that it is probable, that the 

 infects, obferved in the puftules of the itch, and in the (tools of 

 dy fenteric patients, are the confequences, and not the caufes of 

 thefe difeafes. And that the fpecific medicines, which cure the 

 itch, and lues venerea, as brimftone and mercury, act only by in- 



creafing 



