All evil, including disease, entered the world through sin

Wherefore as by one man sin entered into this world, and by sin death; and so death passed upon all men, in whom all have sinned. 

- Romans 5, 12

And God chastises those whom He loves:

“And therefore we also having so great a cloud of witnesses over our head, laying aside every weight and sin which surrounds us, let us run by patience to the fight proposed to us:
Looking on Jesus, the author and finisher of faith, who having joy set before him, endured the cross, despising the shame, and now sitteth on the right hand of the throne of God.
For think diligently upon him that endured such opposition from sinners against himself; that you be not wearied, fainting in your minds.

“For you have not yet resisted unto blood, striving against sin:
And you have forgotten the consolation, which speaketh to you, as unto children, saying: My son, neglect not the discipline of the Lord; neither be thou wearied whilst thou art rebuked by him.

“For whom the Lord loveth, he chastiseth; and he scourgeth every son whom he receiveth.
Persevere under discipline. God dealeth with you as with his sons; for what son is there, whom the father doth not correct?

“But if you be without chastisement, whereof all are made partakers, then are you bastards, and not sons.

“Moreover we have had fathers of our flesh, for instructors, and we reverenced them: shall we not much more obey the Father of spirits, and live?

“And they indeed for a few days, according to their own pleasure, instructed us: but he, for our profit, that we might receive his sanctification.

“Now all chastisement for the present indeed seemeth not to bring with it joy, but sorrow: but afterwards it will yield, to them that are exercised by it, the most peaceable fruit of justice.”

Hebrews 12:1-11

You don’t want to be a bastard, do you? No, I didn’t think so.