The Dreel are a parasitic alien hive-mind from The Return of Nathan Brazil, book four in the Well of Souls series by Jack Chalker.
Specifically, they are described as viruses which induce intelligence in their host, merging with the host's personality (it is unclear if they require already-intelligent hosts, or if an infection can become sapient all on its own), and altering it to serve the greater Dreel hive.
So, how close to that can we get actually get with a plausible biological organism? An Anatomically Correct Dreel should
Be an intracellular parasite of some sort--not necessarily a strict virus, but extra brownie points for actual viruses or viroids.
Give the host the ability to recognize other Dreel.
Either alter the psychology of the host to be altruistic towards other Dreel, or (for extra brownie points) cause the host to become intelligent in the first place.