Leprosy seems to be used in the Bible in a unique sense as an illustration of sin. For one thing it is not said to be "healed" by Jesus but "cleansed". I think the reason is because, like sin, the effect of leprosy seems to always be that it separated one from the covenant blessings and in particular it kept them from the temple and God's people so that they were ceremonially unclean and so unable to serve and worship God. Whether it was Hansen's Disease or not and therefore deadly or some kind of contagious rash has been debated. One reason is because we don't read of people dying from it and needing healing but declared unclean and needing cleansing. In this sense it depicts one aspect of sin. It contaminates everything it touches and ruins one's ability to serve the Lord.
It is interesting to compare some parallels between sin and Hansen's Disease which is what we usually think of when we think of leprosy. Scientists have come to realize that the disease doesn't harm by causing the extremities to rot but that it causes numbness in the extremities and this in turn causes the real harm of leprosy.
In one case the man who is credited with understanding how leprosy works relates an instance which helped him understand leprosy. He was trying to turn a key in an old rusty lock and was unable to do so. Along came a boy of about ten he knew who was afflicted with leprosy and he asked if he could try. To his surprise the boy instantly turned the key and opened the lock. Upon further examination he realized that in the process of turning the key he had ripped open his finger all the way down to the bone but was unaware of it because his leprosy had destroyed his ability to feel any sensation. Another account tells of a man who had gone blind due to leprosy. For years he would wash his face with a washcloth dipped in water but didn't realize that the water was scalding hot. So eventually it destroyed his eyesight. One might step on a nail but because he feels no pain doesn't treat the wound and so it becomes infected and instead of healing it just gets worse and worse, the whole foot starts to rot and death can only be the eventual result.
It isn't hard to connect the dots from how leprosy works to one way that sin can destroy us. Sin's primary side effect is for us to love self above all else including God. Even the most mature saint battles constantly to put the honor of Christ first in everything he does and decision he makes. Like leprosy, sin or pride gets in the way of properly evaluating things we come into contact with. Much like the drug addict or alcoholic who is so infatuated with the physical pleasure of the drug or drink that he becomes insensitive to what it is slowly doing to his body and his life.
I think this can be applied to countless ways sin eats away at our lives. Here is a spouse who because of sin becomes insensitive to the needs of his or her spouse. In his daily interaction with his wife he fails to speak to her and treat her with the love God demands. Such things cause the relationship to weaken until one day he realizes that his marriage is ruined. How many then use the excuse of a bad marriage to justify adultery which just makes the whole problem worse. It has always amazed me how people will allow problems between each other to go on for years causing all kinds of unnecessary friction instead of dealing with them early on before things get intolerable. But it is pride and laziness and a general lack of any real concern for the Lord and each other that allows us to be numb to what is going on around us. And then one day we wake up and wonder why our families and friends and the church isn't what it ought to be or at least why we don't seem to be able to get along with people as we ought.
The Bible says that we are to glorify the Lord in everything we do. In other words, everything we come into contact with is to be used for one purpose, the Lord's service. Sin's tendency is to cause us to see things as how they immediately benefit me! And we can rest assured that this will cause us to use things wrongly and be harmed by them instead of using them properly and benefiting from them.
Perhaps how we deal with trials is a good example. When we understand that they are from the Lord to shape us into useful servants and produce godliness in our lives we can endure them in light of this and by the power of God conquer them. They become tools for our good. But when our sinfulness has us consumed with our immediate comfort then we are easily reduced to complaining and bitterness and they render us incapable of victory. Too much of this kind of spiritual apathy and pretty soon our lives can become open sores where everyone can see the results of sin instead of the image of Christ.