Lev 13:2 - Defiling disease
Leprosy was often thought of as a judgement of God on an individual person for a specific sin, hence the reason there was a law that separated out those who had it. Anyone who had a defiling disease wasn't allowed in fellowship with those who didn't. Again, this is Old Testament stuff, so it's an outward sign of what we now deal with inwardly (since the New Testament came into play). If you're clean you cannot bring into your inner being that which is unclean.
Created about 1 year ago