Ye know how through infirmity of the flesh I preached the gospel unto you at the first.
Note 6 at Ga 4:13: Some people have taken Paul's mention of an "infirmity of the flesh" here, and his reference in Ga 4:15 to the Galatians' willingness to pluck out their own eyes for him (see note 8 at Ga 4:15), as referring to the sickness that Paul called his "thorn in the flesh" in 2Co 12:7. However, as pointed out in note 14 at 2Co 12:7, Paul's "thorn in the flesh" was not sickness.
The assumption that this verse and Ga 4:15 refer to some rare Aramaic eye disease could only be made if someone is already disposed to believe that Paul had a chronic sickness. These passages certainly do not provide evidence for that conclusion on their own.
A much more obvious interpretation is that Paul was referring to some of the physical effects of a stoning he received while in Galatia. In Ac 14:19, Paul was stoned and left for dead (see note 3 at Ac 14:20). This happened in Lystra (Ac 14:8), one of the main cities of the region of Galatia (see Life for Today Study Bible Notes, Introduction to Galatians, The Recipients of Paul's Epistle "Galatians"). Paul was either dead or so close to death that his persecutors thought he was dead. Certainly, he had cuts and bruises all over his body as he preached to the very people to whom he was writing this letter. It would not be unthinkable that he had received injuries to his eyes, which he was referring to here. He did say that this infirmity was "at the first," implying that this was not a permanent, chronic problem but a temporary thing that had healed. In Ga 4:15, Paul may have been using a figure of speech just to emphasize that these Galatians were willing to do anything for him at one time (see note 8 at Ga 4:15).
Paul was referring to injuries he had sustained from the stoning to make the point that in the beginning, the Galatians hadn't despised him but had received him as they would an angel of God. Why had they changed? Paul hadn't changed. The Gospel hadn't changed. It was the Galatians who were inconsistent. He was putting them in remembrance of their original reception of him to rekindle their love for him and to cause them to submit themselves once again to the Gospel that he preached.