I OWN it cannot be demonstrated, that when the _Plague_ makes great Ravage in any Town, the Number of Sick shall never be great enough to load the _Air_ with infectious _Effluvia_, emitted from them in such Plenty, that they may be conveyed by the Winds into a neighbouring Town or Village without being dispersed so much as to hinder their producing any ill Effects; especially since it is not unusual for the _Air_ to be so far charged with these noxious _Atoms_, as to leave no Place within the infected Town secure: insomuch that when the Distemper is at its Highth, all shall be indifferently infected, as well those who keep from the Sick, as those who are near them; though at the beginning of a _Plague_ to avoid all Communication with the Diseased, is an effectual Defence. However, I do not think this is often the Case: just as the _Smoak_, with which the _Air_ of the City of _London_ is constantly impregnated, especially in _Winter_, is not carried many Miles distant; though the Quantity of it is vastly greater than the Quantity of infectious _Effluvia_, that the most mortal _Plague_ could generate.
BUT, to conclude what relates to the _Air_, since the ill Qualities of it in these _Northern_ Countries are not alone sufficient to excite the _Plague_, without imported _Contagion_, this shews the Error of a common Opinion, countenanc'd by Authors of great Name[60], that we are necessarily _visited_ with the _Plague_ once in thirty or forty Years: which is a mere Fancy, without Foundation either in Reason or Experience; and therefore People ought to be delivered from such vain Fears. Since the _Pestilence_ is never originally bred with us, but always brought accidentally from abroad, its coming can have no relation to any certain Period of Time. And although our three or four last _Plagues_ have fallen out nearly at such Intervals, yet that is much too short a Compass of Years to be a Foundation for a general Rule. Accordingly we see that almost fourscore Years have passed over without any Calamity of this kind.
THE _Air_ of our Climate is so far from being ever the Original of the true _Plague_, that most probably it never produces those milder infectious Distempers, the _Small-Pox_ and _Measles_. For these Diseases were not heard of in _Europe_ before the _Moors_ had entered _Spain_: and (as I have observed in the _Preface_) they were afterwards propagated and spread through all Nations, chiefly by means of the Wars with the _Saracens_.