Disease is, of course, a consequence of the fall. Infected individuals and homes have to be removed from God's presence, but thankfully He provides purification rituals to cleanse and restore them. We, too, must have our impurities removed through the cleansing of the Son in order to restore us to right relationship with the Father. Our sin separates us from Him, but when we turn to Christ in repentance, we are cleansed and reunited with Him.
Chapter 15 ensured that the Israelites would separate sexual intimacy from worship, unlike the pagan religions around them.
It is important to note that these ritual or ceremonial impurities were not sinful or morally wrong (unless you refused to undergo the prescribed purification ritual in disobedience). Rather, these rituals pointed to a deeper and more important holiness, living with moral purity in every aspect of life. Sacrifices, however, removed both moral defect and ritual impurity.
Thankfully, we no longer need to abide by the ritual or ceremonial laws, and even more thankfully, Jesus' sacrifice enables us to experience forgiveness from sin in a way only pointed to in the OT.
Tomorrow's Bible In a Year Passage: Leviticus 16-18