4. Leprosy can spread on the individual, and so can sin.
It is seen in Leviticus 13:5 that leprosy can spread on the individual - "And the priest shall examine him on the seventh day [after being isolated]; and indeed if the sore appears to be as it was, and the sore has not spread on the skin, then the priest shall isolate him another seven days." Although there was little they could do to stop it, they were still instructed to note any expansion of the leprosy during that week. Spiritually speaking, Hebrews 3:13 teaches that it is possible for one to be "hardened through the deceitfulness of sin." In other words, although any sin is bad and is to be avoided, it is possible for sin to spread or grow on the individual (thus becoming even worse). Becoming hardened in sin is a process, but, thankfully, such a state can be avoided. No one will live sinlessly (cf. I John 1:8), but those who are wise will confess their sins to God promptly and turn from them before the wickedness grows in strength and makes a slave of the individual.
5. Leprosy can be spread to others by close association, and so can sin.
Leviticus 13:45,46 states - "Now the leper on whom the sore is, his clothes shall be torn and his head bare; and he shall cover his mustache, and cry, 'Unclean! Unclean!' He shall be unclean. All the days he has the sore he shall be unclean, and he shall dwell alone; his dwelling shall be outside the camp." Although the reason why is not explained, it is understood that certain types of leprosy were contagious through close contact. Cannot the same thing be said regarding sin? I Corinthians 5:6 - "Do you not know that a little leaven leavens the whole lump?" Later, in the same book, Paul declared that evil company corrupts good morals (cf. 15:33). If those we associate with regularly are corrupt, their sins will likely rub off on us. Those who are wise will avoid close contact with those who have communicable diseases. But, even more importantly, they will avoid close company with those living in sin (to help minimize the influence of worldliness and the temptation to embrace the same sinful behavior).
6. Leprosy causes separation, and so does sin.
One consequence of leprosy under the Old Testament was that the leper was required to dwell alone outside the camp. He was unclean and had to announce such whenever other people approached him. Thus, leprosy resulted in social separation. In a similar manner, sin causes spiritual separation from God. Consider Isaiah 59:2 - "Your iniquities have separated you from your God; and your sins have hidden His face from you, so that He will not hear." God hates sin, and He is not in close fellowship with unforgiven sinners. The separation caused by sin can only be removed through the atoning blood of Christ (i.e., by means of salvation by grace through faith; cf. Rev. 1:5; Eph. 2:8).
7. Leprosy can be manifested by means of different body parts, and so can sin.
Leviticus 13 identifies several body parts explicitly where leprosy might manifest itself (e.g., skin, head, or beard). Indeed, it was possible for leprosy to break out "all over the skin" from head to foot (Lev. 13:12). Regarding sin, although many sins originate in the heart (cf. Mark 7:21-23), it is possible for sin to reveal itself via various parts of the body. For example, a thief will use his hands to steal, a liar will use his tongue to deceive, a drug addict will use his nose to snort illegal substances, an adulterer will use his sexual organs for illicit activities, etc. Jesus acknowledged in Mark 9:43-48 that it was possible for one to sin via his hands, feet, and eyes. One who wants to enter the kingdom of God must be willing to make any sacrifice--no matter how costly--to overcome sin. No body part (or type of sin) is worth eternal destruction!
We will contemplate some more of these parallels in our next lesson.