A degenerative neurological illness of Vulcans passed on through mind melds performed by poorly trained melders, heavily stigmatised in the twenty-second century, which T'Pol contracted from Tolaris.
In the era of Enterprise, the Vulcan authorities regard mind melding as a deviant practice of a small minority, and Pa'nar Syndrome as the disease those people deserve. T'Pol is infected when the emotional Vulcan Tolaris forces a meld on her in the episode Fusion.
In Stigma, Phlox quietly seeks research on the condition from Vulcan doctors at a medical conference, and they work out who his patient is. T'Pol refuses to save her career by explaining that the meld was forced, because that would endorse the prejudice. A sympathetic doctor, himself a melder, speaks up for her.
The three-part story that ends with Kir'Shara overturns the old view. Melding is part of Surak's true teaching, the syndrome is caused by improper technique, and T'Pau cures T'Pol with a meld of her own.