The Trill are a unique joined species from the Alpha Quadrant composing of a vermiform symbiont and a humanoid host. As a joined life-form, the Trill enjoy a special relationship with past and future. The symbiont lives many lifetimes with the benefit of remembrance, knowing that in all likelihood they will survive beyond the lifetime of any individual host.
The relationship between the Trill symbiont and its humanoid host is complex. A joined Trill integrates the personalities of the host and the symbiont, while retaining memories and personalities of all the other hosts with whom the symbiont joined with previously.
Coping with so many different elements can be difficult. For this reason, host candidates are screened very carefully as the host must must have a strong desire to lead a full life and the will to pursue that life, otherwise he or she takes the risk of being overwhelmed by the symbiont’s previous personalities.
Not all Trill are chosen to be joined with a symbiont. Every year sees only five thousand accepted into the initiate program and only about three hundred accepted for joining. Hosts are seen as a link in the chain of a symbiont’s life; each link must be strong and unique.
The goal of a symbiont is to live a rich and varied existence through their various hosts. Once joined, the host and the symbiont become biologically interdependent at which time non can really live for any amount of time without the other. The resulting life form can be considered to be an entirely different individual, although it retains the memories of both life forms.