Sickness is meant to make us repent and break off our sins. If we will not hear the voice of mercies, God sometimes makes us hear through suffering.
(Paraphrased from unknown author)
Sickness is sometimes not just the result of genetics, injury, or poor diet. Often there seems to be no reason we can discover. It’s in those times that we do well to consider what might be in our conduct and thoughts that is displeasing and harmful.
Is any one of you sick? He should call the elders of the church to pray over him … If he has sinned, he will be forgiven. Therefore confess your sins to each other and pray for each other so that you may be healed. (James 5:14-16)