Psalms 91:6

“Nor for the pestilence that walketh in darkness, etc.] Some think, and not without cause, that what is figuratively expressed in the preceding verse is here explained; and, indeed, the “pestilence” may well be called the “terror by night”: the name of the plague, at a distance, is terrible; the near approach of it is more so; when it enters a country, city, or town, what fleeing is there from it? and in the night season it is more dreadful than in the day; not only to think of it in the gloomy watches of the night, but to see the vast numbers carried out to be interred, and to hear the dismal cry, Bring out your dead: and so it is here said to “walk in darkness”; in the darkness of the…”

“Nor for the pestilence - The plague or pestilence was common in Oriental countries. That walketh in darkness - Not that it particularly comes in the night, but that it seems to creep along as if in the night; that is, where one cannot mark its progress, or anticipate when or whom it will strike. The laws of its movements are unknown, and it comes upon people as an enemy that suddenly attacks us in the night. Nor for the destruction - The word used here - קטב qeṭeb - means properly a cutting off, a destruction, as a destroying storm, Isaiah 28:2; and then, contagious pestilence, Deuteronomy 32:24. It may be applied here to anything that sweeps away people - whether storm, war, pestilence, or…”

“Darkness - Invisibly, so that we can neither foresee nor prevent it. Psalms 91:12”

“91:6 The reference to disease represents any kind of affliction.”

“pestilence: Psa 121:5, Psa 121:6; Exo 12:29, Exo 12:30; Kg2 19:35 destruction: Num 16:48; Sa2 24:15; Mat 24:6, Mat 24:7; Co1 10:3-10 Psalms 91:7”