THE present article, from the learned Bishop of Avranches, if not a valuable, forms at least an ingenious speculation.
Neither naturalists or physicians have informed us what is the cause which renders contagious so many diseases, while others are not in the least infectious. The gout, the gravel, the epilepsy, the apoplexy, are not caught by frequenting the company of the diseased; but the plague, the dysentery, the itch, the bloody-flux, occasion frequently terrible ravages by their infection. This is very probably the fact. It may be said, in general, that all contagious diseases produce worms, which are contained in ulcers, pustules, or pimples, either internal or external, some less and some more, and of different kinds. We shall not here examine the cause of the production of these worms; but their effect is common and unvaried, and sometimes visible. It is also well known that these worms, by undergoing a revolution, which in them is natural, change into the fly state, and become gnats; this is done, in a short time, and in infinite numbers. As soon as these flies, imperceptible by their diminutive size, can lift themselves by their wings, they take their flight. They are then scattered abroad; and, entering the bodies of men by respiration, they infuse that poison by which they are engendered, and communicate that corruption from whence they have sprung.
It is thus great fires have been found very serviceable in public contagions: kindled in divers places, they have, as many imagine, purified the air. The air is, indeed, purified, but not in the manner generally supposed, by rarifying and changing its composition, but in burning and consuming these flying gnats with which the air is filled; and which, attracted by the light of the flames, mix with them, and are destroyed in the same manner as moths are by a candle. An opposite cause produces also the same effect; I mean, a sharp frost, that kills and destroys these terrible insects, if not entirely, at least the greater part: for it has been known, that so great have been their numbers, that many have escaped the rigours of the frost, and have continued the infection; as it happened some centuries back, in the dreadful plague, which desolated Denmark, and the neighbouring countries.