We can say, in one sense, that according to the Bible, all sickness has its ultimate origin in sin because human suffering stems from the fall and the sin of Adam and Eve (Genesis 2:15-17; Romans 1:28-32). But that's not the way some people think of sin causing sickness. They believe a specific sin can cause a particular illness.
The Bible does show that there can be a simple cause-and-effect relationship between sin and sickness. But the Bible doesn't claim that sin always leads to illness or that every illness has its cause in some sin. The Bible does teach that disobedience to God can lead to sickness that ultimately is of supernatural origin.
For example, "If you do not obey the LORD your God and do not carefully follow all his commands and decrees I am giving you today ... The LORD will plague you with diseases until he has destroyed you from the land you are entering to possess. The LORD will strike you with wasting disease, with fever and inflammation, with scorching heat and drought, with blight and mildew, which will plague you until you perish" (Deuteronomy 28:15, 21-22).
Several accounts are given in which an illness is sent as a punishment for specific sins (e.g., Exodus 4:11; Leviticus 26:16; Deuteronomy 32:39; Numbers 12:9-10; 2 Chronicles 7:13; 21:14-15).
Jesus himself made a strong connection between sin and sickness, in at least one man's situation, when he healed the man at the pool in Bethesda (John 5:1-15). He basically equated forgiveness with healing, although his emphasis was on the fact that only God brings about either.
As Jesus bid farewell to this healed man, he made the clearest connection between sin and sickness, warning, "See, you are well again. Stop sinning or something worse may happen to you" (John 5:14).
The clear implication is that if this man sinned again (possibly in some specific way well known to the man), something worse than thirty-eight years of crippling illness would befall him.
This incident may have left the disciples (and readers of John's gospel) wondering if most illnesses result from sin - especially since John later warned that some sins lead to death and others do not (1 John 5:16).
And, two believers, Ananias and Sapphira, were struck dead because of a specific sin (Acts 5:1-11). In addition, some Corinthian believers got ill and died because of their sin (1 Corinthians 11:27-34). Little wonder that some believe the Bible teaches that when people sin, bad things happen.
Today when someone gets seriously ill, has a tragic accident, or is told their disease will be fatal, it is common to ask, "Why? What have I done wrong?" However, it is neither biblical nor logical to believe that a simple one-to-one cause-and-effect relationship exists between sin and illness.
Why? God related to the ancient nation of Israel in significantly different ways than he relates to people today.
Israel entered into an agreement with God in which they knew they would be blessed if they obeyed God - and swiftly punished for disobedience. God told them, "See, I set before you today life and prosperity, death and destruction" (Deuteronomy 30:15). He then told them to choose which they wanted.
The Christian church today does not have such a covenant with God. And, we all know from experience that God does not cause people to be ill every time they sin. If he did, we would be struck by an illness or suffering every time we did anything wrong - in word, deed, or thought.
Another reason we can be confident that not all illness is sent by God as a punishment is that Jesus tells us this is not the case:
As [Jesus] went along, he saw a man blind from birth. His disciples asked him, Rabbi, who sinned, this man or his parents, that he was born blind?"
"Neither this man nor his parents sinned," said Jesus, "but this happened so that the work of God might be displayed in his life." (John 9:1-3)
Jesus is very clear: we should not assume that someone's sickness occurred because the person (or the parents) sinned.