A rare neurological disease affecting Vulcans over two hundred years old, which destroys emotional control and can project the sufferer's feelings telepathically onto others; it afflicts and finally kills Sarek.
Bendii Syndrome is introduced in the Next Generation episode Sarek. The ambassador comes aboard the Enterprise to complete a treaty with the Legarans, and the crew begin lashing out at one another, ending in a brawl in Ten Forward. Beverly Crusher traces the violence to Sarek, whose failing control is spilling out telepathically.
The disease is diagnosed by culturing tissue from the metathalamus and has no cure. To get through the talks Sarek mind-melds with Picard, who carries the full force of the Vulcan's emotions while Sarek borrows his discipline.
By Unification, two years later, the illness has left Sarek bedridden and raving at home on Vulcan, with only moments of clarity. He dies of it during that story.