A virus affecting the empathic abilities of older Betazoids, causing them to project their own amorous feelings onto people around them, as Lwaxana Troi does on Deep Space 9.
Zanthi fever is the explanation for the chaos of the Deep Space Nine episode Fascination. Lwaxana Troi arrives during the Bajoran Gratitude Festival, intent on consoling Odo, and suffers a series of sharp headaches.
Each time, someone nearby is seized by a sudden passion for a person they already find faintly attractive. Jake Sisko pursues Kira, Bareil pursues Dax, Dax pursues Benjamin Sisko, and Kira and Bashir cannot keep away from each other.
Bashir identifies the cause once Lwaxana's own symptoms are examined. The condition is not serious; a simple treatment cures her, and the effects on everyone else fade within a couple of days.