From The Bird Flu: A Classical Perspective, Gabriel Weiss, 2007, p. 10
The context in which the concept of 癘 was used in ancient texts reveals something of the general understanding of the term at the time. For instance, in the early cosmological text the 淮南子 Huainanzi, a philosophical classic from the 2nd century BCE, we read in Chapter 5, “Time’s Lessons 時則訓”, the line
When spring’s mandate is expressed, then insects and larvae cause great loss, the water springs became salty and dried up, and the people experience many acute illnesses and pestilences 疾癘.
 John S. Major, Heaven and Earth in Early Han Thought: Chapters Three, Four, and Five of the Huainanzi (Albany: SUNY Press, 1993).