Neural parasite (or bluegill) were an alien species that attempted to infiltrate the Federation's Starfleet Command by physically infiltrating high-ranking admirals within the organization. Their plans were thwarted by Captain Jean-Luc Picard and Commander Will Riker. However, the mother creature sent a message which Data identified as a beacon, indicating that their return would only be a matter of time. (TNG: "Conspiracy")
One of the bluegills had infiltrated Teero Anaydis in 2374. This allowed him to communicate telepathically with the Spawn Mother. Presumably, the bluegills were using The Circle as a cover for their own activities suggesting they had overcome the weakness of not knowing about their hosts' pasts. Presumably, this was achieved by temporarily relinquishing control of their host bodies. ("Divided Loyalties", "Faces in the Crowd")
The bluegills had the ability to cause the deaths of their hosts when they felt the hosts were in danger of capture. This was the case when co-conspirators of Teero were captured by teams from Deep Space 9 and the USS Lambda Paz. Preliminary autopsies were inconclusive. The parasite that inhabited Lambda Paz engineering technician Saul Mavik caused its host's death after fleeing the reactor room of Starbase G-6 to allow Kelsen Daron to carry out his mission: to destroy the starbase and all surrounding ships. Upon vacating its host, this parasite was incapacitated and taken aboard the Lambda Paz for further study.
Bek Gillen served as a host for one of the bluegills. His editorial program was an apparent method for the Spawn Mother to carry out her plans for Bajor. After the near destruction of Starbase G-6, Bek attempted to disassociate himself from the perpetrators, one of whom was inspired by Bek's warnings, by urging his viewers not to resort to causing such destructive methods in rise up against what he perceived as Federation oppression. ("Faces in the Crowd")