It's cool that for a Trill to receive a symbiont they need to undergo a surgical procedure. Like there's something in Trill anatomy that allows the worm to survive in their tummies (and they become intertwined in a way that kills the host when it's removed) and store the memories of the host as well as pass those memories on to future hosts, but it seems to not be some symbiosis that evolved naturally. It must have been discovered at some point by the Trill (presumably?? All the worms can do is swim around right)
The procedure would probably have been pretty dangerous before the development of sophisticated medicine. Maybe it was only discovered after Trill medical technology had reached a certain advancement where you could slice someone open without having them bleed to death or die of infection. Or maybe Trills just aren't suspectable to those things in the same way humans are.
And there's also some kind of cultural/spiritual element to it because of the way they can do rituals to summon up past hosts. I wonder if the incantations an stuff (which aren't translated by the UT) are technically necessary or if it's more a traditional practice and there's some other physical thing going on?