Dirty Beginnings
The Symbiote started its perceived existence as a virus used to eliminate rather rowdy Captains (see: Star-Crossed Lovers).
Energy Supply
It lives off of suffering. When people die, their energy is displaced and some of it seeps into the beemspace realm. The Symbiote can siphon some of this energy to sustain itself. Captains are particularly tasty, as are starships with koveran drives.
So why would a creature that functions in space filled with koverans need koveran energy from the outside to live? Well, the keyword is live. Koveran energy on its own isn’t alive, but when given a taste of the life energy of the fallen or suffering, it can emulate that to prolong its own ethereal existence.
How is this done?
The Symbiote will use the gap between beemspace and realspace to inhabit the minds of those closest to beemspace. People with neural links are particularly attractive.
Once it gets a foot into the door of one’s mind, it will influence the individual, attempting to cause as much suffering as it possibly can. Fawkes’ sinful past and Martock’s later actions are examples of what a Symbiote will get drawn to and ultimately embrace.
Prevention
As of right now, the only proven artificial deterrence to the Symbiote is an inhibition syrum created by Chiron (see: Chapter 6: Life).
Queens can also deter the Symbiote somewhat, but their affects as of yet are not well catalogued.
The best prevention, of course, is to stay away from death and suffering, and not be in a position where your actions can cause death and suffering… but what fun is that?!
Ascension:
When a Symbiote draws enough power, it will take a realspace form in an attempt to cause more damage. This is called a Koverax, and there are very few instances of this actually happening (see: Chapter 16: Martock).
Death: As of right now, a Symbiote cannot be completely destroyed, since even in its weakest form it will eventually start to grow and influence those around it (see: Nyx, Erl, and the Symbiote).