“But a greater difficulty than that of all persons not being equally susceptible of the infection arises from the cessation of the plague, at a period when the supposed contagious effluvia, preserved in apparel, furniture, and other fomites, at the end of a pestilential season, must be allowed to exist, not only in a much greater quantity than can be supposed to be at once accidentally imported by commerce, but in a state also of universal dispersion over the city: the fact, however unaccountable, is unquestionably certain; the distemper seems to be extinguished by some cause or causes equally unknown as those which concurred to render it more or less epidemical in its advance and at its height. In Europe something may be ascribed to the means employed for the cleansing of houses and goods supposed liable to retain the latent seeds of infection; but, at Aleppo, where the distemper is left to take its natural course, and few or no means of purification are employed, it pursues nearly the same progress in different years: it declines and revives in certain seasons, and, at length, without the intervention of human aid, ceases entirely.” On this we shall remark in general, that the failure of contagion in some cases to produce the usual effects may proceed from some constitution of the body, disposing it not to allow the cause to produce its usual effects at one time, though at another, the constitution may be so far changed as very readily to admit it. This opinion has been very generally received among medical people, who have, to this singularity of constitution given the name of _idiosyncrasy_. It is, however laughed at by Mr. McLean. “As the fact (says he) cannot be denied, that a great majority have escaped after contact with persons ill of diseases supposed to be contagious, attempts may perhaps be made to account for it by supposing a certain peculiarity of constitution, which exempts from, or disposes to, disease. Is it the many who escape that have this happy peculiarity of constitution; or the few who are seized that are so unfortunate as to possesses it? The former are evidently too numerous to admit such an hypothesis. The property must therefore, I conclude, be given to the latter. But a child here and there is exempted from small-pox, although exposed to its contagion. In order to preserve a consistency, this fact must be accounted for by the same or another peculiarity of constitution. Peculiarities of constitution, then, exempt from contagion in one case, and dispose to it in another; and thus a term, which in reality means nothing, may be made to account for any thing. For my own part I confess my inability to comprehend any other _peculiarities_ of _constitution_, or _idiosyncrasies of habit_, than what are constituted by the different degrees of health and disease; the different states of the excitability.”