"For ere this [the opening of Pandora's jar] the tribes of men lived on earth remote and free from ills (kakoi) and hard toil (ponoi) and heavy sickness (nosoi) which bring the Keres (Fates) upon men; for in misery men grow old quickly. But the woman took off the great lid of the jar (pithos) with her hands and scattered all these and her thought caused sorrow and mischief to men."
Now, that might sound like a bunch of mumbo jumbo, so I will summarize. The nosoi were the spirits of illness, plague and disease. Hesiod describes the nosoi escaping from Pandora's jar, and like Elpis (Hope), they were probably personified to a certain degree. However, in most Homeric literature it is the arrows of the gods Apollon and Artemis which bring plague, rather than a band of daimones. The Roman equivalents of the Nosoi were Morbus, Lues, Pestis, Tabes and Macies.
So now you have been offered the incredibly short version of how illness truly came to plague humanity.
Bonaparte Visiting the Pesthouse in Jaffa, Antoine-Jean Gros, 1804, Oil on Canvas, Muse' de Louvre