Af-Kelt was a form of time sickness unique to El-Aurians. It occurred when the timeline had been disturbed in some way, and the El-Aurian subsequently encountered echoes of their own future. Distance from the locus of the timeline disruption offered no protection from the effects.
In 2024, Jean-Luc Picard induced Af-Kelt in Guinan in an effort to prove to her both that he knew her in the future and that something was seriously wrong with the timeline without causing further disruption. (PIC: "Watcher")