It was not generally known that Odan was a Trill, a member of a joined species. The symbiont known as Odan was relatively unharmed, but the humanoid host was fatally injured. Commander William Riker volunteered to serve as temporary host for the symbiont long enough for the ambassador to mediate the peace talks. Following the negotiations, the symbiont was syccessfully transferred into a new host body, Kareel, and returned to her homeworld on a Trill vessel. During the mission, Odan became romantically involved with Beverly Crusher, who in the end rejected Odan, because she couldn't keep up with the changes. ("The Host")

from the Star Trek Encyclopedia