The Symbiont was a sentient vermiform lifeform from the planet
Trill. It was transferred from one humanoid Trill host to
another upon the death of the host. Humans could also be used as
a host to a Symbiont, however the Symbiont could not remain in
their bodies for long without physically harming the Human host
(TNG: "The Host"). The Symbiont took with it all memories of the
previous Trill hosts. Upon joining, the new Trill host gained
these memories. The Symbionts lived in the Caves of Mak'ala on
Trill and were cared for by the Guardians. They communicated
with each other via electrical impulses transmitted through the
milky water they inhabited. The Symbiosis Commission oversaw the
joining of Trill and Symbionts. The general belief in the 24th
century was that one in every 1,000 Trill was suitable for
joining; however, the number was nearer to 50% of the
population. This was a closely guarded secret because every year
about 5,000 Trill became initiates while only about 500
symbionts became available. (DS9: