Galatians 4:13 As you know, it was because of an illness that I first preached the gospel to you.
The “illness” Paul mentions here is never specifically identified. On the basis of Galatians 4:15, and later on in 6:11 (where Paul identifies the handwriting on the letter as his own, in such large letters) many scripture scholars believe that this was some sort of problem with his eyes, or that Paul struggled with poor vision. Some scholars suggest something more serious, such as malaria or epilepsy. There is no mention of illness in the Acts 13 & 14 account, so we can only speculate as to what this means. The literal Greek word here for “illness” or “infirmity” (as the KJV translates it) could mean a physical ailment, or it could mean wounds, impliedly the result of physical persecution. Whatever it was, it was obviously serious enough to cause Paul to stay over in the Galatians’ region for some time in order to recuperate.
Another angle to consider is the ancient Greek’s attitude towards physical illness or infirmity. The stoic philosophers believed that illness should not affect one’s ability to function, so if we are to take the accounts of Acts 13 & 14 in tandem with this passage, Paul still actively functioned in his ministry, despite his illness. That would have scored him points with the “Greek” people among the Galatian congregations. As a side note, at least one of the commentaries I have consulted notes that southern Galatia (the area Paul was traveling through in Acts 13 & 14) would have had the ideal climate for recovery from illness. Indeed, this region was famous through the ages because of its “health benefits.” God knew what he was doing by having Paul “lay over” with the Galatians.