Leprosy, a word that the bible uses to refer to various infections of the skin, posed a major concern amongst the ancient Israelites. Chapter 13 of Leviticus provides methods to diagnose and treat leprosy.
But verses 12 and 13 (KJV) present something strange:
If a leprosy break out abroad in the skin, and the leprosy cover all the skin ... from his head even to his foot ... and ... if the leprosy have covered all his flesh ... he is clean.
What? This sounds like the most severe form of leprosy, one that covers the entire body. Yet a person in that state should be deemed "clean"? Many bible commentators have puzzled over this seemingly contradictory pronouncement. But it's important to realize that, in the bible, "leprosy" also sometimes represents the state of "sinfulness."
So the passage -- which is every bit as directed to us as it was to the ancient Israelites -- seems to be stating that once we recognize out complete and total state of sinfulness, then (and only then) are we ready to receive God's grace. If we perceive any part of ourselves -- even one square inch of ourselves -- to be righteous, then we haven't really reached that state of spiritual humility required by God.
This is very difficult for us! We tend to take pride in certain things. We tend to celebrate our "goodness." Like the Pharisee in chapter 18 of Luke (KJV), we tend to say "God, I thank thee, that I am not as other men." But we must eliminate those impulses and see ourselves for what we really are: totally leperous.