A corrupted State of Air is without doubt necessary to give these _Contagious Atoms_ their full Force; for otherwise it were not easy to conceive how the _Plague_, when once it had seized any Place, should ever cease, but with the Destruction of all the Inhabitants: Which is readily accounted for by supposing an Emendation of the Qualities of the Air, and the restoring of it to a healthy State capable of dissipating and suppressing the Malignity. On the other hand it is evident, that _Infection_ is not received from the Air it self, however predisposed, without the Concurrence of something emitted from _Infected_ Persons; because, by strictly preventing all Intercourse of Infected Places with the Neighbourhood, it may be effectually kept from spreading: Whereas the least Wind must necessarily convey whatever noxious Quality resides in the Air alone, even to a great Distance. Of this we have had a fresh Proof in the present unhappy _Plague_ in _France_, which, by keeping careful Guard, was confined for a considerable Time within the Walls of _Marseilles_; so that none of the adjacent Villages suffered any thing by it; till at length some Persons finding Means to escape carried the Infection along with them. And we find, they have been able, by the like Care, still to restrain it within moderate Bounds. This is the Manner by which Infectious _Effluvia_ are generated; The Way, by which a sound Person receives the Injury, I suppose most commonly to be this. These _Contagious_ Particles being drawn in with the Air we breath, they taint in their Passage the _Salival_ Juices, which being swallowed down into the Stomach presently fix their Malignity there; as appears from the _Nausea_ and _Vomiting_, with which the Distemper often begins its first Attacks. Though I make no Question but the _Blood_ is also more immediately affected by hurtful Particles being mixed through Inspiration with it in the Lungs. The third Way, by which we mentioned _Contagion_ to be spread, is by _Goods transported from infected Places_. It has been thought so difficult to explain the Manner of _this_, that some Authors have imagined _Infection_ to be performed by the Means of _Insects_, the _Eggs_ of which may be conveyed from Place to Place, and make the Disease when they come to be _hatched_. As this is a supposition grounded upon no manner of Observation, so I think there is no need to have Recourse to it. If, as we have conjectured, the _Matter of Contagion_ be an active Substance, perhaps in the Nature of a _Salt_, generated chiefly from the Corruption of a _Humane Body_, it is not hard to conceive how this may be lodged and preserved in soft, porous Bodies, which are kept pressed close together.