And, as Thucydides shows,Cf. Thuc. ii. 4, 7. that calamity which, at the beginning of the Peloponnesian war, harassed the Athenians with a grievous kind of sickness, gradually crept all the way from the torrid region of Africa and laid hold upon Attica.

Others believe that when the air, as often happens, and the waters are polluted by the stench of corpses or the like, the greater part of their healthfulness is spoiled, or at any rate that a sudden change of air causes minor ailments.

Some also assert that when the air is made heavy by grosser exhalations from the earth, it checks the secretions that should be expelled from the body, and is fatal to some; and it is for that reason, as we know on the authority of Homer, i. 50, οὐρῆας μὲν πρῶτον ἐπῴχετο καὶ κύνας ἀργούς. as well as from many later experiences, that when such a pestilence has appeared, the other animals besides man, which constantly look downward, are the first to perish.