Charles H. Spurgeon on Psalms 91:6

The pestilence that walketh in darkness; the destruction that wasteth at noonday. The description is equally forcible and correct. The diseases of all hot climates, and especially where vegetation is highly luxuriant, and marshes and miry swamps are abundant, as in the wilderness here referred to, proceed from the accumulating vapours of the night, or from the violence of the sun's rays at midday. The Beriberi of Ceylon, the spasmodic cholera and jungle fever of India, and the greater part of the fevers of intertropical climates, especially that called the yellow fever, chiefly originate from the first of these —"the pestilence that stalks in darkness"; while sunstrokes or coups de soleil, apoplexies, inflammations of the brain, and liver complaints of most kinds, proceed from the second, "the destruction that wasteth at noonday." And it is in allusion to this double source of mischief that the psalmist exclaims most beautifully on another occasion, Ps 121:6: "The sun shall not smite thee by day, nor the moon by night." And hence the Israelites were miraculously defended against both during their passage through the wilderness by the pillar of a cloud in the daytime, to ward off the solar rays; and by the pillar of fire by night, to dissipate the collecting vapours, and preserve the atmosphere clear, dry, and healthy. J. M. Good.