Charles Ellicott Commentary Leviticus 13:12-13

"And if the leprosy break out abroad in the skin, and the leprosy cover all the skin of [him that hath] the plague from his head even to his feet, as far as appeareth to the priest; then the priest shall look; and, behold, if the leprosy have covered all his flesh, he shall pronounce [him] clean [that hath] the plague: it is all turned white: he is clean." — Leviticus 13:12-13 (ASV)

And if a leprosy break out abroad. —There were, however, two phases of this returning disease that exempted the patient from uncleanness. If the leprosy suddenly covered the whole body, so that the patient became perfectly white and there could be no appearance of live flesh, then he was clean. This indicated the crisis, as all the diseased matter brought to the surface in this way formed a scale, which then dried and peeled off.