And the History of the most terrible of all the Plagues, that ever were in these Parts of the World, which was that in the Year 1349, gives a manifest Proof from whence all Europe may trace the Origine of these Evils, viz. from Asia: for [[8]]This taking its Rise in China in the Year 1346, advanced through the East Indies to Syria, Turkey, Ægypt, Greece, Africa, &c. In 1347 some Ships from the Levant carried it to Sicily, Pisa, Genoa, &c. In 1348 it got into Savoy, Provence, Dauphiny, Catalonia, Castile, &c. In 1349 it seized England, Scotland, Ireland, and Flanders; and in the next place Germany, Hungary, and Denmark; and in all these Countries made most incredible Havock.
But to return to the Consideration of the Air, which we left in a putrid State: It is to be observed, that Putrefaction is a kind of Fermentation, and that all Bodies in a Ferment emit a volatile active Spirit, of Power to agitate, and put into intestine Motions, that is, to change the Nature of other Fluids into which it insinuates it self.
It were easy to shew from the best Theory of Fevers[[9]], how the Alterations made in the Blood this Way will favour Pestilential Diseases, by rendring the Body obnoxious to them: But the Digression would be too great.
THIS is one step towards Contagion. The next, as it seems to me, proceeds after this Manner. The Blood in all Malignant Fevers, especially Pestilential ones, at the latter End of the Disease, does like Fermenting Liquors throw off a great Quantity of active Particles upon the several Glands of the Body, particularly upon those of the Mouth and Skin, from which the Secretions are naturally the most constant and large. These, in Pestilential Cases, although the Air be in a right State, will generally infect those, who are very near to the sick Person; otherwise are soon dispersed and lost: But when in an evil Disposition of This they meet with the subtle Parts, its Corruption has generated, by uniting with them they become much more active and powerful, and likewise more durable and lasting, so as to form an Infectious Matter capable of conveying the Mischief to a great Distance from the diseased Body, out of which it was produced.
They who know what strange Attractions and Combinations are made by volatile Spirits will understand this Reasoning; especially if they consider, how easily all kinds of Effluvia are diffused in a warm Air, such as we have described an Infectious one to be; and further, of how penetrating a Force the finest Parts of Animal Juices are; of which the strange Stench of a mortified Limb, upon a Body yet living, will convince any one.
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.
We all know how long a time Perfumes hold their Scent, if wrapt up in proper Coverings: And it is very remarkable, that the strongest of these, like the Matter we are treating of, are mostly Animal Juices, as Mosch, Civet, &c. and that the Substances found most fit to keep them in, are the very same with those which are most apt to receive and communicate Infection, as, Furrs, Feathers, Silk, Hair, Wool, Cotton, Flax, &c. the greatest Part of which are likewise of the Animal Kind; which Remark alone may serve to lead Us a little into the true Nature of Contagion.