He distinguishes carefully _pestilence_ from _epidemy_ and _endemy_.

The common notion of the two first accords exactly with that of an epidemic constitution, for both consist, according to him, in an unknown change or corruption of the air; with this difference, that pestilence calls forth diseases of different kinds; epidemy, on the contrary, always the same disease.

As an example of an epidemy, he adduces a cough (influenza) which was observed in all climates at the same time without perceptible cause; but he recognised the approach of a pestilence, independently of unusual natural phenomena, by the more frequent occurrence of various kinds of fever, to which the modern physicians would assign a nervous and putrid character.

The endemy originates, according to him, only in local telluric changes--in deleterious influences which develop themselves in the earth and in the water, without a corruption of the air.

These notions were variously jumbled together in his time, like everything which human understanding separates by too fine a line of limitation.