“Atque ea vis omnis morborum, pestilitasque, Aut extrinsicus, ut nubes nebulæque superne Per cœlum veniunt, aut ipsâ sæpe coorta _De terrâ surgunt_, ubi putrorem humida nacta est, Intempestivis pluviisque, et solibus icta.” (De Rerum Nat. vi, 1100.) Silius Italicus appears to refer an epidemical fever to the same cause. (xiv.) In his work, on the ‘Varieties of Fever,’ Galen expresses his opinion on this subject very fully. He remarks that an atmosphere of a hot constitution, such as generally prevails at the time of the rising of the dog-star, having been inhaled by the heart, increases the heat in it, from which it is diffused all over the system, and enkindles a febrile affection. In pestilential constitutions, he adds, it is principally by the respiration that the disease is contracted, although sometimes it may arise from the fluids of the body being disposed to putrescency, which is increased by the condition of the atmosphere; but for the most part epidemical complaints derive their origin from the atmosphere being tainted with putrid exhalations. The putridity of the atmosphere may be occasioned either by a multitude of dead bodies which have not been burnt, as is apt to happen in wars, or it may arise from the exhalations of certain marshes or lakes in the summer season, or sometimes the inordinate heat of the atmosphere may give rise to them, as happened in the case of the Athenian plague, according to the testimony of Thucydides. And here we must digress to remark, that Homer evidently ascribes the plague which attacked the Grecian army to the great heat of the sun. See the Commentary of Eustathius on the beginning of the Iliad; Ammianus Marcellinus (xix); Heraclides Ponticus (Allegor); and Macrobius (Saturn. vii, 5.) Galen, however, inculcates that the constitution of the atmosphere alone is not sufficient to produce disease without a peculiar disposition of the body to admit it; for that, otherwise, all without exception would be seized with the prevailing epidemic. This leads him to give directions to correct the intemperament of the body when it is such as disposes it to be readily affected by the constitution of the atmosphere. His directions are similar to those of our author. Besides the causes of epidemical diseases which we have mentioned, he states that unwholesome food and drink may sometimes, though rarely, give rise to them. Of this he relates a striking instance. (De Rebus boni et mali Succi, c. i.) He remarks that the most common epidemical diseases are pestilential fevers. We shall have occasion to state his opinion of them in the next Section. Of the Greek authors posterior to Galen, Oribasius and Aëtius give the same account of epidemical diseases as our author; and the others either do not treat of them at all, or class them with the subject of our next Section.