Leviticus 13:22

And if it spread much abroad in the skin, then the priest shall pronounce him unclean: it is a plague.

The verse belongs to the laws on leprosy and uncleanness for cleansing in Leviticus 12-15, where a skin disease spreading widely abroad in the skin is judged by the priest, who is to pronounce the person unclean, for it is a plague. This visible, bodily uncleanness had to be removed before the sanctuary's distinct cleansing by blood, showing that ritual uncleanness and the sanctuary's moral defilement were different matters. The leprosy instructions are admirable sanitary guidance and precautions against contagious or infectious diseases and unsanitary conditions, part of the laws of sanitation promulgated through Moses. Those laws still stand at the head of all sanitary law, and their supreme excellence proves divine guidance and inspiration, while observing them preserves life and prevents disease.

The spreading leprosy and the priest's declaration of uncleanness portray sin as a plague, and leprosy in its utmost loathsomeness was made a fitting type and prefigure of sin as a dreadful condition and mortal sin from which people need healing. Like leprosy, sin is a universally prevalent, terrible moral disease that never gets well of itself and is incurable by human means. It consumes its victim, has its chief seat within, polluting the source of life, as well as bringing outward shame. Like leprosy, sin spreads and is contagious and unclean: the sinner contaminates the very atmosphere and introduces pollution where all before was comparative purity, and the sinner's society leads others to sin, so such influences are more to be feared than the infection of the leper.

Cleansing comes through the help of the great Physician, whose blood cleanses from sin, and healing of leprosy displays his power to heal. The picture teaches that sin cannot be put away while pride and the old self remain; people must be humbled, put off the old man, and put on the new. The continued prevalence of leprosy in the Holy Land in the same form as in scriptural days bears witness to scriptural truth.