The Trill have a symbiotic relationship with another being however only few Trills are able to join with the symbiont creature due to the scarcity of the symbionts. This leads to many tests and screening to make sure that only the best in society are able to join.
The Trill were created late during the television Show Star Trek: the Next Generation. In the Next Generation episode Trill had large protruding foreheads and no spots. This was changed during the creation of the show Star Trek: Deep Space Nine. The reason for this was the actress Terry Ferrell did not look good in the protruding forehead so the look of the Trill was changed.
The Trill race went through a dramatic change when a group of terrorist led by Verad Kalon
created a virus to kill all joined Trill.