Leviticus 13:20
And if, when the priest seeth it, behold, it be in sight lower than the skin, and the hair thereof be turned white; the priest shall pronounce him unclean: it is a plague of leprosy broken out of the boil. And if, when the priest seeth it, behold, it be in sight lower than the skin, and the hair thereof be turned white; the priest shall pronounce him unclean: it is a plague of leprosy broken out of the boil.
The test is priestly: when the affected spot appears lower than the surrounding skin and its hair has turned white, the priest must pronounce the person unclean, for this marks leprosy broken out of a boil. It belongs to the laws regulating uncleanness and leprosy and to the law of cleansing, in which bodily uncleanness had to be removed and the person made visibly clean before the sanctuary's deeper cleansing by blood, the priest's work being to identify the visible signs and pronounce uncleanness. It is part of the sanctuary's yearly cycle of service, which typified Christ's once-for-all ministry.
It belongs also to sanitary instructions about leprosy and other contagious diseases that give valuable guidance about infection. Those Mosaic sanitary laws still stand at the head of sanitary law, and faithful observance of them prevents disease and preserves life, showing that divine inspiration guided Moses. The leprosy here is a horrible and loathsome disease that fittingly types and prefigures sin, including mortal sin.
Like leprosy, sin is a terrible moral disease that infects all humanity, cannot be cured by human means and never gets well of itself. It spreads through contact and contaminates the very atmosphere, consumes its victim, has its chief seat within while polluting the source of life and bringing outward shame. It introduces pollution where there was comparative purity, and leads others into sin through society and influence, so it is more to be feared than a leper's infection. Sin is the transgression of the law and all unrighteousness, embracing every kind of evil.
Unlike literal leprosy, sin can be cleansed through Christ the great Physician by His blood, and leprosy as presented through Moses became a means by which Jesus Christ showed His power to heal. Healing from the sin thus prefigured requires the death of pride and the old self, and without that humbling change a person still needs conversion. The cleansing laws to which this belongs exemplify the cleansing God's people need, calling them to obey health reform and other cleansing teachings so as to become clean vessels fit to receive the latter rain and not to fall away as ancient Israel did. The same type of leprosy still prevails in the Holy Land as in scriptural days, bearing witness to scriptural truth.