82 FOURTH REPORT — 1834. 



second in order of sequence, whether smallpox or cowpox, as- 

 sumes a modified, and generally a much milder form*. There 

 can be little doubt, however, that those two diseases are essen- 

 tially the same. We have no evidence that any one specific dis- 

 ease affords a security against any other, which is distinguished 

 from it by marked characters and a different succession of sym- 

 ptoms. Neither smallpox nor cowpox gives a durable protection 

 against measles, hooping-cough, or scarlatina. 



XXVII. It is in few instances only that two contagious poi- 

 sons act together upon the human body, producing simultane- 

 ously two distinct maladies. Scarlatina has been known to su- 

 pervene on typhus ; and hence the precaution, in some fever 

 hospitals, of distinct wards for those two diseases. Smallpox 

 and cowpox may coexistf; so also may cowpox and measles J; 

 but smallpox and measles are incompatible at the same time. 

 Mr. Hunter inoculated for smallpox a child who, as afterwards 

 appeared, had been previously exposed to the infection of mea- 

 sles. The measles appeared and completed its course, before the 

 inoculation took effect, after which the smallpox began, and 

 passed through its usual stages §. Two similar instances are 

 related by Dr. Darwin, in both of which the smallpox, after 

 being suspended by the measles, exhibited an unusually mild 

 character 1 1. 



XXVIII. A certain duration of exposure to contagious ema- 

 nations is essential to their full effect. This is precisely analo- 

 gous to what happens with respect to noxious gases, which may 

 be breathed in mixture with common air, for a few moments, 

 M'ithout injury. On this subject Dr. Haygarth's observations 

 establish the conclusion, that air weakly impregnated with small- 

 pox or typhus contagion, may be breathed for a long time, and 

 air strongly charged with either, for a short time, with equal im- 

 punity^. Medical practitioners who have sustained no injury 

 from visits of ordinary duration, have been infected after staying 

 unusually long in the apartments of persons suffering imder con- 

 tagious fevers. A very dilute contagion, however, is known to 

 disorder the health, when it does not produce the whole of the 

 morbid phsenomena in their usual degree and order of suc- 

 cession. 



XXIX. We have no observations suiSiciently correct to enable 



* For the fact that cowpox is milder after smallpox, see Jenner's Tract, 1798, 

 p. 18. 



f Adams On Morbid Poisons, p. 398. J Jenner's Tract, 1799, p. 63. 



§ Hunter On the Blood, &c., Introduction. || Zoonomia, §. xxxiii. 



«i Letter to Dr.Percival, p. 41. 



