The parasites were small alien beings who could take over the bodies of those they had infected, who included Admirals Quinn, Aaron and Savor, as well as Captain Tryla Scott and Lt. Commander Remmick. When Picard's old friend Captain Keel attempted to warn him of a possible conspiracy in the upper brass of Starfleet, his ship was subsequently destroyed. Picard returned to earth, and the infected Admiral Quinn came aboard the ship. While aboard, Quinn attempted to infect Dr. Crusher, but Riker caught his attention first, and he tried to infect him 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. Upon inspecting Quinn's body, Crusher found that Quinn's body was his own, but he wasn't in control of his actions.
Down on Earth, Picard had dinner with Remmick, Scott and the Admirals, who ate bowls of mealworms, revealing themselves to him. Riker entered the room, and was apparently infected as well. Riker looked as though he was going to eat the worms, but suddenly pulled out a phaser, and he and Picard successfully subdued all of the infected members of Starfleet in the room, causing their parasites to leave their bodies. They followed a parasite that came out of Quinn to a room where Remmick was sitting. Remmick, it turned out, was infected with the mother creature of the parasites. The mother creature speaking through Remmick boasted it wanted peaceful coexistence, but Picard and Riker turned phasers on it, successfully killing it, though Remmick was killed as well, along with all other parasites on Earth.
Back on the Enterprise, Riker revealed that Dr. Crusher had put a fake gill on the back of his neck to simulate the infection and fool the other parasites. As the Enterprise moved on into space, Data theorized that Remmick had been transmitting a homing beacon before he died and that should the parasites receive it, they would know where to find Earth.
It was later discovered that the parasites were actually genetically altered Trill symbionts. Many centuries ago the Trill colony on Kurl had performed alterations on the Trill symbionts to cure them of a disease that had already killed ten percent of the colony. The alterations changed the shape of the symbiont and also positioned the altered symbionts on the brain stem instead of in the abdomen after research indicated that it would improve communication between host and symbiont.
However the virus adapted and after years of facing the disease and the alterations the symbionts had gone mad, becoming a race of psychopaths. The Trill committed genocide on Kurl via remote orbital bombardment, however they failed to wipe out all the parasites. A few parasites survived and set themselves to destroying the Trill.
Some years after the attempt to take over Starfleet the Bajoran First Minister Shakaar Edon stumbled on a parasite spawnmother while on a goodwill tour of the Federation. She took over Shakaar and began moving to take over the Bajoran system. Her children infected people all over Bajor and Deep Space Nine. The parasite infestation was discovered when Shakaar's body was assassinated, by then his personality was long gone.
The spawnmother had left Shakaar's body prior to the assassination, leaving the body under the control of a soldier parasite. She was preparing to do a mass spawning in a temple housing the Bajoran Orbs when Kira Neyrs opened all nine orbs up in front of the spawnmother. The prophets sent her to a realm occupied by the Jem'Hadar who had been taken and placed there by the prophets earlier in the war. The Jem'Hadar promptly slaughtered the spawnmother. Meanwhile the entire parasite colony active in the Bajoran system fled and died soon afterwards, ending the parasite threat.
Bajor entered the Federation after the ending of that parasite threat. It was not known if other spawnmothers still existed, waiting for the right moment to strike. The spawnmother that had infected Shakaar believed herself to be the last spawnmother in existence.
In Star Trek Online they are given the name "Bluegill" and are revealed as bio-engineered creations of the Solanae .