A flying parasite was roughly disc-shaped, about thirty centimeters in diameter and two to four centimeters in height. The edges were thin and yellowish; towards the center, the creature was thicker and redder. Occasionally, they pulsated. Parasites had no detectable external or internal organs, and they emitted buzzing sounds similar to bees. Spock described a parasite as "resembling, more than anything, a gigantic brain cell." They were very resistant to phaser fire, even sustained and at high power levels.
These parasites were capable of clumsy flight. They attacked by making physical contact with a target and stinging it as does a bee. The stinger injected a strand of tissue that infiltrated the victim's nervous system very rapidly, entwining about the nerves. Leonard McCoy described this entwining as "far, far too involved for conventional surgery to remove." Once the parasite infiltrated a host, it pressured the host to obey its commands by inflicting enormous pain. There seemed to be some level of pain even when the host obeyed, but the creature could increase the pain it inflicted to bring an uncooperative host to heel. Spock, himself, Later, was able to end the pain through mental discipline and convince the crew to let him collect one of the parasites to run tests on. However, for the others, the continuous painful stimulation led to insanity and death.
Infected victims cooperated in working towards the parasite's objectives. This, and their enormous resistance to harm, led Spock to theorize that all of the parasites were parts of a single organism. The parasites were so alien that Spock also theorized their origin was a place where different physical laws applied, outside of the Milky Way Galaxy. (TOS: "Operation -- Annihilate!")
A linear progression of insanity marked the spread of the parasites through space. According to 23rd century archaeologists, the ancient civilizations of the Beta Portolan system were the first victims. In the 21st century (around the 2060s, or two hundred years before the Deneva incursion), Levinius V was attacked. After that, Theta Cygni XII, and in 2265, Ingraham B succumbed. A ship from Ingraham B next brought the parasites to Deneva colony, where after eight months of infestation the crew of the USS Enterprise managed to destroy them in 2267.
The breakthrough in defeating them came when James T. Kirk, whose own older brother George Samuel Kirk and sister-in-law Aurelan Kirk had both been killed by the parasites, theorized that they were vulnerable to visible light. Clues suggested this: a Denevan declared himself "free" as his ship approached the system's sun, and the parasites remained within buildings, in shadows.
The first attempt at a cure using this approach used exposure to a general spectrum of light at 1,000,000 candles per square inch and it proved effective at killing a parasite. Spock volunteered for the next test to see if the creature's tissues could be destroyed within the body. Seeing as how the eyes of the Denevan colonist could not be shielded, Spock refused eye protection for the test and was temporarily blinded, although the foreign matter was also destroyed. Unfortunately, the test proved to be premature as analysis of the creature's remains revealed that ultraviolet light was sufficient. Initially, when Spock wrongly believed himself permanently blinded, he described the trade as "equitable" - perhaps the most cogent comment on how much pain the creatures could inflict anyone could have made. However, it turned out that Spock's inner eyelids, which instinctively closed to protect his eyes from strong light exposure, had prevented permanent damage to his optic nerves.
A constellation of 210 ultraviolet satellites was placed in permanent orbit 72 miles around the planet. The light they generated reached all parts of Deneva, destroying the entire infestation. (TOS: "Operation -- Annihilate!")
Background information Edit
The script of the episode refers to the aliens as "Things" and "Creatures" only, while the name "Denevan neural parasite" was used by the Star Trek Encyclopedia (3rd ed., p. 112) despite Spock indicating that the mass insanity originated in the Beta Portilin system. The name "neural parasite" was simply used in the reference guide, Star Trek: Aliens & Artifacts.
Finally, the display graphic in O'Brien's school, which was adapted from the Star Fleet Medical Reference Manual, called the single "flying parasite", genus "Blastoneuron."