Ephesians 2 verse 3 to 5
Job 14 verse 1
Romans 8 21
It is true that God did not create us to be sick or ill. We were created to have a relationship with God. However, because sin entered the world through Adam, we and all of creation have been placed under a curse. The curse of sin brought decay, sefishness, and all manner of difficulty that we must live with here on earth. As Job says in 14:1 of a mans life, it is short and full of trouble.
After we become Christians we are not exempted from the temporal consequences of sin on such as sickness, death, trial and challenge. After all we too are children of wrath and sons of disobedience according to Ephesians 2 when talking about the old nature we war against.
Hebrews chapter 12 verse 5 talks about the discipline of God. He judged us as Sinners in need of Jesus' sacrifice on the cross, He judged us as sons/daughters with his discipline.
He sometimes allows us to have problems in our lives to teach us to think and bring us back to Christ. 2nd Corinthians chapter 12 verse 4
Paul for instance, had a stake driven in him (thorn in the side), a malady, and after 3 extended sessions he was told "My grace is sufficient for you". In this case we see there is a dependency factor to trial that keeps us depending on God.
This is what Romans 8:28 means when it says "And we know that for those who love God all things work together for good, for those who are called according to his purpose." This is saying that even our difficulties are seen by and pass through the hand of God.
We must remember that even hardship of our lives has a purpose. We must therefore having thanks to God in every situation. In this way we change people from looking into themselves and working on themselves to be stronger in the face into ministers who lift up others in similar circumstance to show the glory of God even in the midst of trial.