Leviticus 13:3

And the priest shall look on the plague in the skin of the
flesh
Whether it be a swelling, scab, or a bright spot that appears, and judge of it by the following rules, and none but a priest might do this:

and [when] the hair in the plague is turned white;
it arising in a place where hair grows, and which hair is not naturally white, but of another colour, but changed through the force of the plague; and there were to be two hairs at least, which were at first black, but turned white; so Jarchi and Ben Gersom: and these hairs, according to the Misnah ^F5, must be white at bottom; if the root (or bottom) is black, and the head (or top) white, he is clean; if the root white, and the head black, he is defiled; for hairs turning white is a sign of a disorder, of weakness, of a decay of nature, as may be observed in ancient persons:

and the plague in sight [be] deeper than the skin of his flesh;
appears plainly to view to be more than skin deep, to have corroded and eat into the flesh below the skin:

it [is] a plague of leprosy;
when these two signs were observed, hair turned white, and the plague was more than skin deep, then it was a plain case that it was the leprosy of which (See Gill on Matthew 8:2) (See Gill on Matthew 8:3) (See Gill on Luke 5:12). This was an emblem of sin, and the corruption of nature, which is an uncleanness, and with which every man is defiled, and which renders him infectious, nauseous, and abominable; and of which he is only to be cured and cleansed by Christ, the great High Priest, through his blood, which cleanses from all sin. The above signs and marks of leprosy may be observed in this; the white hair denoting a decay of strength, see ( Hosea 7:9 ) may be seen in sinners, as in the leper, who are without moral and spiritual strength to keep the law of God, to do anything that is spiritually good, to regenerate, renew, convert, and sanctify themselves, or to bring themselves out of the state of pollution, bondage, and misery, in which they are; and, like the leprosy, sin lies deep in man; it is in his flesh, in which dwells no good thing, and in which there is no soundness; it does not lie merely in outward actions, but it is in the heart, which is desperately wicked; for the inward part of man is very wicked:

and the priest shall look on him, and pronounce him unclean;
and so should be obliged to rend his clothes, make bare his head, put a covering on his upper lip, and cry, unclean, unclean; dwell alone without the camp, and at a proper time bring the offering for his cleansing, and submit to the several rites and ceremonies prescribed, ( Leviticus 13:45 Leviticus 13:46 ) ( 14:1-57 ) .