|A neural parasite|
|An infected Gregory Quinn|
|The telltale spike of infection|
|The mother creature|
Neural parasite aliens attempted to infiltrate the Federation's Starfleet Command in 2364, by physically infiltrating high-ranking admirals within the organization. Persons who fell victim to infiltration included Admirals Gregory Quinn, Aaron, and Savar.
Admiral Quinn suspected that something was wrong at the highest levels of Starfleet Command in mid-2364. Seeking allies he could trust, he ordered Lieutenant Commander Dexter Remmick to investigate Captain Jean-Luc Picard to determine his trustworthiness. Remmick's investigation found that, despite a sense of over-familiarity among the USS Enterprise-D's bridge crew, Picard was above reproach. Quinn told Picard of his suspicions and that he wanted to promote Picard to admiral so he could take over as commandant of Starfleet Academy. Picard turned down the offer, believing he could serve Quinn better by remaining as captain of the Enterprise. (TNG: "Coming of Age")
Other high-ranking officers had their own suspicions. Admiral Norah Satie was instrumental in discovering the conspiracy. (TNG: "The Drumhead") Captains Walker Keel, Rixx, and Tryla Scott noticed a pattern of unusual orders and activity coming from Starfleet Command. Officers they had known for years were, in Keel's words, suddenly "bluffing their way through talk of old times," Starbase 12 was mysteriously evacuated for two days without explanation and Onna Karapleedeez, Ryan Sipe, and McKinney had all died under mysterious circumstances.
Quinn and Scott fell victim to the parasites, both becoming hosts, and Remmick became host to the parasites' mother creature. Captains Keel and Rixx were unaware of this and called Picard to a meeting on Dytallix B via a Code 47 signal. They warned Picard of the threat, and to watch his back. Keel's ship, the USS Horatio, was destroyed shortly afterwards, with Keel aboard. This incident, and evidence of a pattern in Starfleet's unusual orders found by Lieutenant Commander Data, led Picard to order the Enterprise to Earth to confront Starfleet Command directly.
Quinn boarded the Enterprise, and while Picard was on the surface Quinn attempted to implant Dr. Beverly Crusher with one of the parasites. Commander William Riker caught Quinn's attention first, and Quinn attempted to infect Riker instead. Riker attempted to fight him off, but was easily overpowered, as were Worf and Geordi La Forge. He was only subdued when Dr. Crusher shot him repeatedly with a phaser set to stun.
Having subdued Admirals Savar and Aaron and Captain Scott, Picard, and Riker tracked one of the parasites to a room where Dexter Remmick, inhabited by the parasite's mother creature, was waiting. Both Remmick and the mother creature were killed by heavy phaser fire at the hand of Captain Picard and Commander Riker, thereby also killing all other parasite infiltrators on Earth. Admiral Quinn, at least, made a full recovery.
The parasites' natural appearance resembles that of light purple, crawling insects. A humanoid lifeform inhabited by one of the parasites displays a noticeable spine-like protrusion from the base of the neck that permits the parasite to breathe. They are apparently incapable of inhabiting Soong-type androids such as Data.
The parasites appeared to be highly intelligent, perhaps more intelligent than Humans, and even cultured, at one point claiming "a love of theatre." The parasites were incapable of retaining either some or all of a host's memories, providing another means through which they could be exposed.
A Human inhabited by the parasites will display increased physical strength – such that the elderly Admiral Quinn was able to easily fight off three much younger men, including even Worf – and vitality; even the host's resistance to phasers is increased. Not realizing that Quinn had been inhabited, Picard had commented that Quinn looked much healthier than the last time they had met.
The parasites are connected to a mother organism, a larger creature likewise capable of inhabiting a humanoid host. The death of this creature seems to result in the deaths of all progeny. (TNG: "Conspiracy")
The neural parasites were designed by Andrew Probert. In his concept designs, the mother creature originally had a head more reminiscent of the smaller parasites. The parasite storyline was not revisited partially due to the negative reaction "Conspiracy" received, as the story was too different from what viewers had come to expect from Star Trek. (The Art of Star Trek) The creatures were later built by Makeup & Effects Laboratories, a company headed by Allan A. Apone. (Star Trek: The Next Generation 365, p. 111)
The writers originally intended the parasites to be agents of the Borg. Due to the Writers' Strike of 1988 as well as budget cuts, the connection between the Borg and the parasites was never established. (Star Trek: The Next Generation Companion)
The neural parasites were (historically) first encountered on a comet near Trill, by then head of Trill Symbiosis Commission Audrid Dax, whose husband was a victim of a parasite, accompanied by Christopher Pike. The Trill authorities classified the knowledge and never released it to public; these events are depicted in the short story "Sins of the Mother" in the anthology The Lives of Dax.
In the Pocket DS9 "relaunch" series of novels, Mission Gamma and the conclusion Unity had the neural parasites return in 2376, shortly after the end of the Dominion War. Shakaar Edon was one of those infiltrated and was assassinated by a Trill delegate to Bajor's entering of the Federation. It is revealed that they are similar to the Trill symbionts, as the husband of Audrid Dax was infected by one on a mission to an asteroid that was expected to pass Trill and showed a symbiont-like lifeform. The parasite fused with the symbiont and ultimately caused the death of both the host and the symbiont, a greater loss.
In the novel Worlds of Star Trek: Deep Space Nine, Volume 2, it was revealed that the neural parasites were genetically created from Trill symbionts in an ancient Trill colony on Kurl; these symbionts were designed to be removed without killing the host and were passed from Trill to Trill. The colony was annihilated by the regular Trill when it was discovered that the fatal combination of a mutated virus and genetic engineering caused the symbionts to go mad.