John 5:12-15 (NKJV)
Then they asked him, "Who is the Man who said to you, 'Take up your bed and walk'?" But the one who was healed did not know who it was, for Jesus had withdrawn, a multitude being in that place. Afterward Jesus found him in the temple, and said to him, "See, you have been made well. Sin no more, lest a worse thing come upon you."
The man departed and told the Jews that it was Jesus who had made him well.
Jesus healed the lame man and left the scene. Later Jesus came to find the man and then told him to stop sinning or a worse thing might happen to him. I love the way Jesus sought this man out to give him the real message. What does it mean to sin no more? This is repentance which is a change of mind and heart about sin. Repentance is turning away from sin and self and turning toward Jesus Christ. After Jesus said this to the man, notice that the man immediately went to tell others about Jesus.
What is the "worse thing" that could have happened to the man? It could be that there is connection between sickness and sin. There are scriptures that would lend support to this idea. Certainly this is a sin cursed world we live in and the perfection of paradise is ruined because of man's sin. However, if this man had not repented of his sin, he would have come under judgment. So too will you and I if we have not repented of our sins.