That the Reasonableness of any Method, which shall be proposed to prevent the spreading of _Contagious_ Diseases, may the better appear, it is necessary to premise somewhat in general concerning _Contagion_, and the Manner, by which it acts. Contagion is propagated by three Causes, the _Air_; _Diseased Persons_; and _Goods transported from infected Places_. We shall therefore first enquire what alteration of the _Air_ it is, that makes it _infectious_; and then, by what Means it communicates its noxious Quality to other Bodies. The ancientest and best Authors of Physick, who lived in a Country more exposed to these Calamities than ours, observed the Constitution of the _Air_, which preceded _Pestilential Fevers_, to be great _Heats_ attended with much _Rain_ and _Southerly Winds_[1]: And one of Them takes particular Notice, that no other than a _moist_ and _hot_ Temperament of the _Air_ brings the _Plague_; and that the Duration of this Constitution is the Measure of the Violence of the Distemper[2]. The Natural History of several Countries confirms this Observation; in _Africa_ particularly, if Showers fall during the sultry Heats of _July_ and _August_, the _Plague_ ensues thereupon, with which whosoever is infected hardly escapes[3]. It has besides been remarked in all Times, that the Stinks of _stagnating Waters_ in hot Weather, _putrid Exhalations_ from the Earth; and above all, the Corruption of dead _Carcasses_ lying unburied, have occasioned _infectious Diseases_. From hence it appears to be a _Concurrence_ of Causes, that produces Diseases of this Kind; which must not only meet, but exert their Force together for a considerable time. And when this happens, their first effect is a Degree of _Stagnation_ in the Air, which is afterwards followed by _Corruption_ and _Putrefaction_. And upon this account it is, that those Countries are chiefly liable to these Calamities, where not only the _Heats_ are very great, and the _Weather_ continues long in the same State; but the _Winds_ (the Use of which is by Motion to purify the Air) do not shift and change so often as they do in Northern Climates. Indeed _Plagues_ seem to be of the Growth of the _Eastern_ and _Southern_ Parts of the World, and to be transmitted from them into colder Climates by the Way of _Commerce_. Nor do I think, that in this _Island_ particularly there is any one Instance of a _Pestilential_ Disease among us of great Consequence; which we did not receive from other _infected_ Places. This I the rather mention, because it is a common Opinion, and propagated by Authors of great Name, that we are usually _visited_ with the _Plague_ once in 30 or 40 Years; which is a mere Fancy without any Foundation either in Reason or Experience: and therefore People ought to be delivered from the Subjection to such vain Fears.