Nimietatem frigoris aut caloris vel umoris vel siccitatis pestilentias gignere philosophi et illustres medici tradiderunt. unde accolentes loca palustria vel humecta tusses et oculares casus et similia perferunt, contra confines caloribus tepore febrium arescunt. sed quanto ignis materies ceteris est efficacior, tanto ad perimendum celerior siccitas. hinc cum decennali bello Graecia desudaret ne peregrinus poenas dissociati regalis matrimonii lucraretur, huius modi grassante pernicie telis Apollinis periere complures, qui sol aestimatur. atque ut Thucydides exponit, clades illa, quae in Peloponnesiaci belli principiis Athenienses acerbo genere morbi vexavit, ab usque ferventi Aethiopiae plaga paulatim proserpens Atticam occupavit. aliis placet auras, ut solent, aquasque vitiatas faetore cadaverum vel similibus salubritatis violare maximam partem, vel certe aeris permutationem subitam aegritudines parere leviores. affirmant etiam aliqui terrarum halitu densiore crassatum aera emittendis corporis spiraminibus resistentem necare non nullos, qua causa animalia praeter homines cetera iugiter prona Homero auctore et experimentis deinceps multis, cum talis incesserit labes, ante novimus interire. et prima species luis pandemus adpellatur, quae efficit in aridioribus locis agentes caloribus crebris interpellari, secunda epidemus, quae tempore ingruens acies hebetat luminum et concitat periculosos humores, tertia loemodes, quae itidem temporaria est sed volucri velocitate letabilis.
(Ammianus Marcellinus, Res Gestae 19.4.2-7)
Philosophers and eminent physicians have told us that an excess of cold or heat, or of moisture or dryness, produces plagues. Hence those who dwell in marshy or damp places suffer from coughs, from affections of the eyes, and from similar complaints; on the other hand, the inhabitants of hot climates dry up with the heat of fever. But by as much as the substance of fire is fiercer and more effective than the other elements, by so much is drought the swifter to kill. Therefore when Greece was toiling in a ten years’ war in order that a foreigner* might not evade the penalty for separating a royal pair, a scourge of this kind raged and many men perished by the darts of Apollo**, who is regarded as the sun. And, as Thucydides shows***, 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**** 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. Now the first kind of plague is called endemic, and causes those who live in places that are too dry to be cut off by frequent fevers. The second is epidemic, which breaks out at certain seasons of the year, dimming the sight of the eyes and causing a dangerous flow of moisture. The third is loemodes*****, which is also periodic, but deadly from its winged speed.
* Paris, the cause of the Trojan War.
** See Iliad, i. 9 ff. and 43 ff. Apollo was angry because the request of his priest was denied. Ammianus rationalizes the myth, attributing the pestilence to the heat of the sun, and likening its rays to the arrows of the god.
*** Cf. Thuc. ii. 4, 7.
**** Iliad, i. 50, οὐρῆας μὲν πρῶτον ἐπῴχετο καὶ κύνας ἀργούς.
(tr. John C. Rolfe, with his notes)