The bluegill, more commonly referred to simply as parasites, were a parasitic genetically modified off-shoot the Trill symbionts created to try and cure the symbionts of a terrible disease. The experiments failed resulting instead in a race of psychopaths which the Trill tried and failed to eradicate. From then on the parasites maintained a hatred for the Trill and by various means attempted to destroy them. It played a major role on the Bluegill infestation of Keroes (one of the battles of the Infection War), which Alexia Lancaster was infected with the parasite and was killed by Marian Hawke after her spawnmother came out of her body
The Parasites were insect-like in appearance and took control of their victims by crawling down his or her throat and tapping directly into their brain and spinal cord. The process was eventually fatal to the victim.
In addition to an impressively fast crawl speed for such small creatures, the parasites were capable of jumping the height of most humanoids to quickly gain access to the mouth. They had a large set of strong pincers which they used to force their way into more resistant hosts.
The parasites shared the same kind of isoboromine neurotransmitter links with their hosts as Trill symbionts, but, unlike Symbionts, they take complete control of their host. The only physical clue to a parasite's existence is a small barb that protrudes from the back of the host's neck (in later incursions the parasites learned to cover that up by plastimasking it onto the host's skin). The barb acts like a gill, expelling spent gases taken from the host's blood absorbed through pores in the parasite's exoskeleton. Regular parasites have neither eyes or a mouth.
A parasite infected host could be detected by changes in behavior. Hosts tended to develop trembling fingers and rapid eye movement as well as a taste for Klingon and Ferengi food, both of which include vermiform invertebrates. The parasites could access the short term memory of the host, but not the long term, which could give them away as a parasite controlled host would be unable to accurately answer detailed questions about the host's own past. However, even once uncovered, the host is virtually immune to phaser stuns due to the parasites ability to neutralize their victims pain receptors and controlling adrenalin levels. This also gives the host increased strength, making subduing a host difficult.