What is the thorn in Paul’s flesh?

Is Paul's "thorn" in his flesh an actual ailment, or a reference to troublesome people “tormenting” his ministry?

Let's begin by looking at the passage.

2 Cor 12:1 Boasting is necessary, though it is not profitable; but I will go on to visions and revelations of the Lord.
2 Cor 12:2 I know a man in Christ who fourteen years ago—whether in the body I do not know, or out of the body I do not know, God knows—such a man was caught up to the third heaven.
2 Cor 12:3 And I know how such a man—whether in the body or apart from the body I do not know, God knows—
2 Cor 12:4 was caught up into Paradise and heard inexpressible words, which a man is not permitted to speak.
2 Cor 12:5 On behalf of such a man I will boast; but on my own behalf I will not boast, except in regard to my weaknesses.
2 Cor 12:6 For if I do wish to boast I will not be foolish, for I will be speaking the truth; but I refrain from this, so that no one will credit me with more than he sees in me or hears from me.
2 Cor 12:7 Because of the surpassing greatness of the revelations, for this reason, to keep me from exalting myself, there was given me a thorn in the flesh, a messenger of Satan to torment me—to keep me from exalting myself!
2 Cor 12:8 Concerning this I implored the Lord three times that it might leave me.
2 Cor 12:9 And He has said to me, “My grace is sufficient for you, for power is perfected in weakness.” Most gladly, therefore, I will rather boast about my weaknesses, so that the power of Christ may dwell in me.
2 Cor 12:10 Therefore I am well content with weaknesses, with insults, with distresses, with persecutions, with difficulties, for Christ’s sake; for when I am weak, then I am strong.

Paul leaves his ailment intentionally vague, probably because he didn’t want to place the focus on himself. His point in writing was that his supreme revelations had the potential to develop within him great pride, which the Lord counteracted by giving Paul an ailment. Ironically, had Paul spoken specifically about that ailment, he would have been acting contrary to the very purpose for which the Lord assigned it to him.

Therefore, Paul never says what the Lord gave him, and if Paul (and the Lord) desired that the specific nature of his “thorn” remain unspecified, then we should make no attempt to determine it either.