John Calvin Commentary Deuteronomy 28:27

"Jehovah will smite thee with the boil of Egypt, and with the emerods, and with the scurvy, and with the itch, whereof thou canst not be healed." — Deuteronomy 28:27 (ASV)

The Lord will smite you with the botch of Egypt. Whether you understand this passage as referring to the extraordinary plagues God inflicted on the Egyptians at the time of His people’s deliverance, or to the ordinary diseases that had previously prevailed among them (though the latter is more probable), Moses still signifies that while the Egyptians were afflicted with these plagues, God’s people escaped them. This distinction was intended to represent His favor more clearly.

For it could not happen naturally that in the same place, diseases from which the Israelites were free would afflict only the Egyptians. Therefore, God threatens that if they should despise His Law, He would deal with them as they had seen Him deal with pagan nations.

And certainly, since God then chose to multiply His people miraculously, it cannot be doubted that He wonderfully privileged them with the bestowal of health and vigor.

It is doubtful whether by diseases of the rectum He means hemorrhoids, prolapsus, or some other secret disease, such as the one that afflicted the Philistines when they captured the ark of the covenant (1 Samuel 5:6).

He then adds other diseases in which special marks of God’s wrath appear. For although these sometimes also affect the children of God, I have still shown elsewhere that the same punishments are dealt out to them respectively, so that they differ widely from each other.

When Job was afflicted with terrible ulcers, so that he became corrupt, he seemed for a time to bear the marks of a reprobate person. However, what in that holy man was an exercise of patience is, in the transgressors of the Law, the just reward of their crimes by the curse of God.