Bluegill

In 2364 a parasitic alien race gained control of much of Starfleet HQ, the crew of the Enterprise-D successfully thwarted there attempt to take over (TNG episode: Conspiracy). Later analysis of there tactics revealed they were placing heavy fire power near to the planet Trill in preparation for an attack.

A century earlier a comet heading towards trill was inhabitant by a parasite which took control of the Trill Jayvi Vod eventually resulting in his death along with three human Starfleet officers also on the expedition. Whilst on the comet it was discovered that the parasites were almost genetically identical to Trill symbiots and they were but the first en route to destroy the Trill and other inferior life froms. The Trill government covered up this incident destroying the comet and allowing the parasite infected Vod symbiot to die rather than be placed in a new host.

The parasites share the same kind isoboromine nuerotransmitter links with there hosts as Trill Symbiots but unlike Symbiots they take complete control of there host. The only physical clue to a parasites existence is a small barb that protrudes from the back of the hosts neck, but in later incursions the parasites had learned to cover that up by plastimasking it onto the hosts skin. The parasites can access the short term memory of the host but not the long term which can give them away. However even once uncovered the host is immune to phaser stuns due to the parasites neutralising there pain receptors and controlling adrenalin levels giving the host increased strength making subduing a host difficult. The parasites can function independently but its thought they prefer to work in hives with a queen telepathically communicating and controlling other 'soldier' parasites and possibly communicating with other queens. The parasites can be removed from hosts but only within three-four weeks of bounding, after which the neurological connections become to complex and the host would be killed, the exception to this is when the queen parasite is killed the soldier parasites release control of there hosts and are relatively easily removed.

A decade after there attempt to take over Starfleet they began another ploy taking other the Bajoran government… (DS9 novel: Mission Gamma, Book 4: Lesser Evil) (DS9 novel: Unity)