Paul himself said it:
2 Corinthians 12:7 KJV
7. And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure.
And it was not sickness: it was exactly what it says it was. "A messenger of Satan to buffet him, let he be exalted above measure.
Nothing to do with sickness. And what did the Lord tell Paul?
Paul asked, and the Lord told Him repeated:
2 Corinthians 12:8-9 KJV
8. For this thing I besought the Lord thrice, that it might depart from me.
9. And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me.
Weymouth even translates the "it" depart as "rid me of him."
2 Corinthians 12:8 Weymouth
8. As for this, three times have I besought the Lord to rid me of him;
Now I realize that some hold that the Lord's "sufficient Grace" somehow equates to a "no." Which is, again, a part of the problem. When people pray for things if they expect His"sufficient grace" to equate to a "no," and they receive exactly what they expect: a no.
In Paul's case, the answer was exactly what the Lord said... "my grace is sufficient for thee. "
We have to ask, "sufficient to do what?"
Obviously, the answer is just a matter of reading comprehension - Sufficient to do what Paul was asking for... to have the demonic spirit depart from him. The Lord told him this three times.
Paul expands on what the "weakness" was. It is persecution that was raised up by the demonic spirit. Again. Word for word from the passage:
2 Corinthians 12:10 KJV
10. Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong.
In the end, the whole point of Paul having to endure these troubles is because of pride. To temper his pride, a messenger from satan followed him around to stir up problems in his wake—nothing to do with sickness. As long as Paul had a pride problem, he would be weakened so that the pride would not be in the way of his ministering.