Look at the Law of Moses in relation to leprosy
Look up the Law of Moses concerning leprosy: Leviticus 13.9-17
Leprosy is seen as being caused by a sin, not an illness, which is why a leper is described as 'unclean'. It was thought at the time that this sin was one that only the Messiah-King could eradicate. Matthew would certainly see this as Jesus effectively stating that he is the Messiah.