77 Do not then fear if your flesh be eaten away, the soul is not consumed. And so David says that he does not fear, because the enemy were eating up his flesh but not his soul, as we read: “When evil-doers come near upon me to eat up my flesh, my foes who trouble me, they were weakened and fell.” So the serpent works overthrow for himself alone, therefore is he who has been injured by the serpent given over to the serpent that he may raise up again him whom he cast down, and the overthrow of the serpent may be the raising again of the man. And Scripture testifies that Satan is the author of this bodily suffering and weakness of the flesh, where Paul says: “There was given unto me a thorn in the flesh, a messenger of Satan to buffet me, that I should not be exalted.” So Paul learned to heal even as he himself had been made whole.
Source: On Repentance (New Advent)