ALL that we know, is this, that the Cause of the _Plague_, whatever it be, is of such a Nature, that when taken into the Body, it works such Changes in the Blood and Juices, as to produce this Disease, by suddenly giving some Parts of the Humours such corrosive Qualities, that they either excite inward _Inflammations_ and _Gangrenes_, or push out _Carbuncles_ and _Bubo's_; the _Matter_ of which, when suppurated, communicates the like Disease to others: But of the manner how this is done, I shall discourse in the following Chapter. CHAP. II. _Of the Causes which spread the Plague._ I HAVE been thus particular in tracing the _Plague_ up to its first Origine, in order to remove, as much as possible, all Objection against what I shall say of the Causes, which excite and propagate it among us. This is done by _Contagion_. Those who are Strangers to the full Power of _this_, that is, those who do not understand how subtile it is, and how widely the Distemper may be spread by _Infection_, ascribe the Rise of it wholly to the malignant Quality of the _Air_ in all Places, wherever it happens; and, on the other hand, some have thought that the Consideration of the infectious Nature of the Disease must exclude all regard to the Influence of the _Air_: Whereas the _Contagion_ accompanying the Disease, and the Disposition of the _Air_ to promote that _Contagion_, ought equally to be considered; both being necessary to give the Distemper full force. The Design therefore of this Chapter, is to make a proper Balance between these two, and to set just Limits to the Effects of each. FOR this purpose, I shall reduce the Causes, which spread the _Plague_, to three, _Diseased Persons_, _Goods transported from infected Places_, and _a corrupted State of Air_. THERE are several Diseases, which will be communicated from the Sick to others: and this not done after the same manner in all. The _Hydrophobia_ is communicated no other way than by mixing the morbid Juices of the diseased Animal immediately with the Blood of the sound, by a _Bite_, or what is analogous thereto; the _Itch_ is given by _simple Contact_; the _Lues Venerea_ not without _a closer Contact_; but the _Measles_, _Small-Pox_, and _Plague_ are caught by a _near Approach_ only to the Sick: for in these three last Diseases Persons are render'd obnoxious to them only by residing in the same House, and conversing with the Sick.