The lepers “stood afar off” because leprosy was a dreaded, loathsome disease for which God gave Moses detailed instructions to deal with it. This was an incurable disease that would eventually disfigure and rot away the body. It was widely known that only God could heal it. So, when Christ healed the leper in Matthew 8:1-4, His divine nature was revealed to many. When the ten lepers saw Jesus, they were likely tempted to rush toward Him to be healed, but they obediently observed the legal distance of 100 paces (Leviticus 13:46; Numbers 5:2; II Kings 5:5).
In the Bible, leprosy illustrates the work of sin. Like leprosy, sin is a vile, contaminating, mortifying, unclean thing. It starts out as a spot that grows and festers until it takes in the whole person, condemning him to death. It is a type of the separation that sin causes, as well as representing how all people are alike in sin: “For there is no difference; for all have sinned and fall short of the glory of God” (Romans 3:22-23).
Martin G. Collins
The Miracles of Jesus Christ: Ten Lepers Healed