Charles Ellicott Commentary Psalms 91:5

"Thou shalt not be afraid for the terror by night, Nor for the arrow that flieth by day;" — Psalms 91:5 (ASV)

Terror by night. —Possibly a night attack by an enemy. (Compare Song of Solomon 3:8; Proverbs 3:23–26.) Compare Milton:

“To bless the doors from nightly harm.”

In this case the arrow flying by day would refer to dangers of actual battle. But it is quite possible that the latter may be merely the Oriental expression for the pestilence, since it is still so called by Arabians. “I desired to remove to a less contagious air. I received from Solyman the emperor this message: that the emperor wondered what I meant in desiring to remove my habitation. Is not the pestilence God’s arrow, which will always hit his mark?”—Quoted in Spurgeon’s Treasury of David, from Busbequin’s Travels.