Worlds of DS9: Trill: Unjoined
On a related note, why would it be so devastating to Trill society if it became known that they have a lot more hosts physically capable of hosting symbionts than they have symbionts? Theyíre a reasonably mature and democratic society, arenít they? Arenít they capable of coming up with an appropriate process for deciding who gets the symbionts, and enforcing that process?
And how many people have known this secret? How did they convince the public (what kind of evidence did they fake) that only very few Trills were capable of hosting, and how did hundreds of people (or was it thousands? or more?) manage to keep such a huge secret without anybody ever letting anything slip?
goes into detail exploring Trill society and answers a lot of these questions. In short, even mature and democratic societies don't stop people being greedy and thinking they deserve symbionts. Verad wouldn't be the only one willing to kill for the social and mental advantage of Joining. Also, Trill society and history is incredibly steeped in keeping secrets and hiding shame, to the point that entire swaths of history are forgotten.
A stork brings them, of course.
A stork symbiote
Symbiotes always seemed kind of a problem to me. They are always shown as needing delicate surgery to install/uninstall, and that just doesn't seem something that would evolve naturally.
Or maybe the symbiotes can
blend with hosts naturally, but the mortality rate was very high so the Trill developed medical methods to increase the survivability of both symbiote and host.
I don't recall if Trill: Unjoined
ever definitively answered this question, of where exactly the symbiotic relationship originated, but I recall there being somewhat conflicting memories among the Annuated. But I'm pretty sure they all point to a biological symbiosis of some sort.