Under the Old Covenant, touching the unclean defiled a person (Leviticus 5:3), but Christ showed that under the New Covenant, this was not so. Instead, evil thoughts, murders, adulteries, fornications, thefts, false witness, and blasphemies are what defile a person (Matthew 15:18-20). Jesus never did any of these evil acts, and contrary to what the Jews thought about touching a leper, He could never be defiled.
However, when we view His touching the leper as a defiling act according to the Old Covenant, it reveals a realistic picture of the distinction between man and God. God put the filthy sins of the world on Christ so that we may be cleansed and forgiven. Christ "who knew no sin [took sin on Himself] that we might become the righteousness of God in Him" (II Corinthians 5:21).
God's power to intervene is apparent in this healing, as the cleansing of the leprosy occurred immediately, instantaneously, upon touching him. If the healing had taken a prolonged time, the world would have had an opportunity to deny that Christ had healed the leper. They would likely have claimed that the natural healing process of the body made him well. Following Jesus' example, the apostles also laid hands on the sick, by which the power of God's Holy Spirit healed them (see Acts 10:38; I Corinthians 12:9).
Martin G. Collins
The Miracles of Jesus Christ: Healing a Leper (Part Three)