There are a few exceptions, but the exceptions prove the rule.
God can cause your cell to develop abnormally to be cancerous when you are adamant about opposing God or your sin is serious. God will give you cancer, even if God forgives you for the serious sin. God is fair. If you have adultery but the Christian who sits next to you at church does not, then it is fair that God gives you punishment that shows that God is fair. You must distinguish between eternal punishment and punishment on earth. Adam and Eve went to Heaven because they are believers, but they were cast out of the Garden of Eden and made to suffer on Earth and get physically sick and experience physical death because they ate something that God told them not to eat. Simple as that. God punishes on earth, even though God may let you into Heaven because of your repentance.
Examples of sins for which you could get cancer as God’s punishment for your sin:
(1) Adultery (having sex with someone who you are not married to)
(2) Having had abortion (killing your unborn child)
(3) Supporting or tolerating homosexuality in society, by voting for politicians who have open platform of supporting pro-gay marriage laws.
God punishes believers for their sins. This is clear in the Bible.
God is fair to those who keep God’s Word. God is fair to those who go against God’s Word. God is fair. God is just.
If you have cancer, you should quietly reflect before God what sin you committed to be punished with cancer. If you did not repent, then you should. But you should know that despite the fact that God will forgive you for your sin when you repent, even if you may go to eternal life in Heaven, you WILL suffer on this earth for sins you committed.
God is fair to those who uphold God’s Law. God is fair to those who oppose God’s Law. God is fair. God is just.