8G FOURTH REPORT — 1834. 



from others ; and infants at the breast show a remarkable in- 

 sensibility to some contagious maladies. 



XXXIV. But of all the circumstances that impart the power of 

 resisting contagion, the most remarkable is the force of habit. In 

 this respect, as in many others, we find a close analogy between 

 ordinary and contagious poisons. Large doses of opium, any one 

 of which would be fatal to an uninitiated person, are habitually 

 swallowed several times daily, by those accustomed to its use. 

 In like manner, medical practitioners and the nurses of the sick 

 breathe, with impunity, contagious emanations to which they 

 are in the daily habit of being exposed. It was remarked by Dr. 

 Ferriar, that the keepers of lodging-houses in Manchester, of the 

 lowest and filthiest kind, from which typhus fever was seldom 

 absent, were untouched by the reeking poison, Avhile the new- 

 comers kept up a constant succession of victims to its effects*. 

 To habit, also, the prisoners, who carried contagious poison in 

 their clothes into a court of justice, owed their own protection. 



XXXV. The immunity acquired by habit is not, however, in 

 all cases either permanent or absolute. 1. Medical practitioners 

 and nurses, who have long discontinued their avocations, have 

 again become liable to be infected by febrile contagionf. 2. Per- 

 sons accustomed to breathe without injury atmospheres impreg- 

 nated to a certain extent with contagion, yield to the influence of 

 stronger doses. The late Dr. Clark, of Newcastle, though ren- 

 dered by constant habit proof against typhus contagion of com- 

 mon strength, caught that disease in a severe form by suddenly 

 undrawing the bed-curtains of a patient, and thus subjecting him- 

 self to a rush of air more than usually pestilential];. 3. Persons, 

 who by habit are enabled to resist one kind of infection, do not 

 on that accovmt enjoy a security against others. Of this, beside 

 many other instances, we have a striking illustration in the havoc, 

 which spread so rapidly among the medical practitioners in 

 Prussia, when Asiatic cholera first appeared in that country §. 



XXXVI. There is reason to believe that contagious poisons 

 may be received into the system, and may remain in it some time 

 without manifesting their usual consequences, until some acci- 

 dental cause calls them into full action, and gives birth to the 

 usual train of symptoms. Circumstances of this kind have been 

 called coxcuBRiNG or exciting causes. Generally speaking, 

 they are identical with those which, acting upon the body before 

 exposure to contagion, are termed predisposing causes, the enu- 



* Ferriar, Medical Histories, vol. i. p. 173. 



+ Haygarth's Letter, pp. 41, 44. % Clark's Collection of Papers. 



§ Dr. Wagner's "^Report of the Cholera in Prussia," Bibl, Brit., No. 51. 

 p. 179; awA^WWrnmn American Journal, vol. .\xv, p. 17!). 



