Generally, all sickness is a result of Adam’s disobedience. “Through one man sin entered the world, and death through sin, and thus death spread to all men, because all sinned,” Romans 5:12. Sickness is part of the dying process.
However, we can contribute to and stimulate our physical weaknesses, or we can have healthy habits which promote wellness. (Sickness cannot be eliminated.) “The merciful man does good to his own soul; but he that is cruel troubles his own flesh…A cheerful heart is a good medicine, but a downcast spirit dries up the bones.” (Proverbs 11:17; 17:22)
The Apostle Paul had eyesight trouble and asked the Lord three times to heal it. (2 Corinthians 12:7-10) The Lord denied his request. Paul concluded his poor eyesight kept him humble and dependent on God. Timothy had stomach infirmities. (1 Timothy 5:23) We don’t know why, but we still believe he was a faithful Christian.
Sometimes God will use physical problems to correct a Christian, but only that person would be able to understand that experience. Others should not judge (Romans 14:4) a person as sinful because he is sick.
What about Jesus healing and forgiving sins such as in Matthew 9? Looking closely at Matthew 9, Jesus forgave the man’s sins, but he still was sick. The healing did not happen at the same time when Jesus pronounced the words, “Your sins be forgiven.” Jesus disconnected the sin from the healing.
In Matthew 9, Jesus was making a point to the Pharisees. Jesus was the representative of God’s mercy and the world’s Redeemer. He had the power to forgive sins. Under the law, the Pharisees had no such authority.
Consider also the story in John 9. The Apostles asked Jesus if the man’s blindness was a result of his parents’ sins or his own. Jesus clearly stated, “Neither…it was so that the works of God might be displayed in him..,” John 9:1-3 (NASB).
Ultimately, God has permitted all sin and sickness; however, it will all work to His glory. “He made known to us the secret of His will. And this is in harmony with God’s merciful purpose for the government of the world…of restoring the whole creation to find its one Head in Christ; yes, things in Heaven and things on earth…” Ephesians 1:9, 10 (Weymouth).
For a more detailed study about the permission of evil, please read the free PDF at the link: "Why God Permits Evil"