Two alien races are living in sybioses. The symbiote is an energy creature of some sort. The host aliens do something violent (maybe an action in a war?) that causes the symbiotes to decide to leave. The hosts are Not Happy.
I only read the first chapter, which included a solitary symbiote that had not left its host, a kind, elderly being on the brink of death. Don't know what happened after that.
This appears to be at least a partial match to Star Surgeon by Alan Edward Nourse. This is a novel of approx 180 pages, published in 1959.
In this book Earth provides all the medical support for the Galactic Confederation, which prior to Earth joining the Confederation was lacking any medical support. A trainee/probationary doctor from the planet Garv attempts to (and succeeds in) joining the medical service provided by planet Earth, despite the opposition of several of the highest ranked doctors. He does so in his final tests, where he and the rest of the crew (2 others, also doctors) on a General Practice Patrol Ship (GPPS) tend to various emergency calls for help. The main message of the story is about the struggle of a stranger trying to be accepted in a foreign society.
In one call, they visit a planet called 31 Bruckner, which as its occupants dying of a plague. The team of Drs on the GPPS go through their standard tests and eventually produce a treatment for the affliction that is causing the plague. However, this treatment actually kills the symbiote (a virus in this case), leaving the host intact. However, the host is normally a harmless and mindless animal, and the symbiote is the intelligent component, and the team have to work out how to save the intelligent component.
