ANSWER: First, let us look at the scriptural passage in which Paul mentions his affliction to his flesh.
7. But in order that I might not be exalted by the abundance of the revelations, there was given to me a thorn in the flesh, angel (messenger) of Satan to buffet me, so that I might not be exalted. 8. I besought the Lord three times for this, that it might depart from me; 9. But He said to me, "My grace is sufficient for you; for My power is made perfect in weakness." (2Corinthians 12:7 - 9, HBFV throughout)
What were the revelations given to the apostle that eventually led to the thorn he received? He states that fourteen years prior (2Corinthians 12:2), in 43 A.D. (one year before his first evangelistic journey), God gave him unique visions of his throne where he heard amazing things he could not repeat (2Corinthians 12:1 - 4).
Sometime after these visions occurred, the apostle began to experience his "thorn" which he calls a "messenger of Satan" (KJV translation). God allowed the devil to inflict him in order to counteract attitudes of spiritual vanity (self-exaltation) he could easily justify by being the recipient of such special revelations (2Corinthians 12:7).
Bible commentaries, in trying to explain what is meant in 2Corinthians 12:7, offer a whole host of possible explanations regarding the thorn Paul said affected his flesh or body. St. Paul the traveler and the Roman citizen states the apostle suffered from bouts of chronic malaria fever. The early Christian author Tertullian thought it was an earache. Chrysostom, considered an early church "father," conjectured it was a reoccurring severe headache (Adam Clarke's Commentary) which is what Jerome, a Catholic theologian, thought as well (Barnes' Notes on the New Testament). Adam Clarke himself thought what bothered him were the FALSE teachers that visited the church in Corinth and taught a perverted gospel (2Corinthians 11:12 - 15). The Expositors Bible Commentary stated that some who had a "competent opinion" on the matter believed the evidence pointed to epileptic seizures..
Although 2Corinthians is silent regarding the extact nature of Paul's thorn in the flesh, the book of Galatians does record some information about an affliction he had to deal with.
13. Now you know that at first I preached the gospel to you in the weakness of the flesh; 14. And the temptation - namely, my trial in my flesh - you despised not, nor rejected with contempt; rather, you received me as a messenger of God, even as Christ Jesus Himself . . . For I bear you witness that, if it were possible, you would have plucked out your eyes and given them to me (Galatians 4:13 - 15).
Jamieson, Fausset, and Brown's Commentary on Galatians 4:13 states that the Greek in the verse means the following.
"'Ye know that BECAUSE of an infirmity of my flesh I preached . . .' He (Paul) implies that bodily sickness, having detained him among them, contrary to his original intentions, was the occasion of his preaching the Gospel to them."
The apostle, during his first evangelistic journey, had an infirmity or "thorn" so severe that it caused a change of plans and forced him to spend time in Galatia. This view is supported not only by the JFB, but also by The Expositors Bible Commentary and other reference works. Conybeare and Howson's classic work on his life states the following regarding his efforts in the province of Galatia.
"St. Paul affectionately reminds the Galatians (Galatians 4:13) that it was "bodily sickness which caused him to preach the Glad Tidings to them at the first." The allusion is to his first visit: and the obvious inference is, that he was passing through Galatia to some other district (possibly Pontus, where we know that many Jews were established), when the state of his bodily health arrested his progress." (The Life and Epistles of Apostle Paul by Conybeare and Howson, Chapter 8).
What exactly was the illness to his flesh that was so severe it made Paul stay in Galatia? Near the end of Galatians, we are given another clue regarding this condition.
11. See with what large letters I have written to you with my own hand (Galatians 6:11).
His reference to a visible infirmity that could cause people to reject him (Galatians 4:14), his acknowledgement that the church would have 'plucked out' their own eyes for him (verse 15), coupled with the physically large letters he used to write the epistle (6:11), points to a disease affecting the eyes. Scofield Reference Notes on Galatians 6 states the apostle likely had ophthalmia (inflammation of the eyes), which made seeing very difficult. Such a disease would have made travel difficult and could have easily distorted his face.
The affliction Paul received in order to keep him humble (2Corinthians 12) and the disease referred to in Galatians 4 and 6 may be related. If they are, we can construct the following basic chronology of events.
The Apostle Paul has visions of heaven in 43 A.D. and shortly afterward receives his "thorn in the flesh," possibly a disease that inflamed his eyes. During his first missionary journey (44 to 46 A.D.), a flare up of his condition becomes so severe that it forces him to stay in the region of Galatia. Believers in the area readily accept him even though his face looks distorted. When he writes the book of Galatians in 53 A.D., he still has the same basic affliction he possessed when he first visited the area. The condition makes it hard to see and causes the apostle to write to the church using large letters.