Deneva Mind Parasite
Deneva mind parasites are organisms that live in large swarms that pray on sapient beings. The monster was featured in TOS "Operation Annihilate!"
The parasite is composed of numerous small, flat organisms. In function, each individual organism acts like a large brain cell. The "cells" communicate by unknown means, allowing all of the organisms to act as a single, large entity. The parasite individuals make a buzzing noise and tend to avoid lighted areas.
They are capable of limited flight, and they attack by adhering to the spinal column of the intended victim and injecting tendrils that wrap around the central nervous system. The tendrils cause excruciating pain, which the parasite can vary in intensity. The gestalt mind of the parasite is able to communicate its desires to the victims, punishing those who try to resist its demands. This can lead to madness and/or death for victims who continue to resist despite the pain. Because of the mental disciplines practiced by Vulcans, they are better able to limit and resist the control of the parasites.
The creatures are not capable of manipulating objects and technology, but through their victims they are able to facilitate the infection and spread their species from planet to planet. How the parasite reproduces was not determined, as Captain Kirk decided that ending the infection was more important then studying its basic biology. However, once they had been brought to Deneva they did spread until they had infected the entire population of the colony.
Cure and Destruction
Due to the destructive and deadly nature of the parasites, Captain Kirk determined that the creatures had to be destroyed. During the initial investigation on the planet Deneva, Mr. Spock was attacked and infected with one of the parasites. Finding a cure to the infection had an additional personal connection with the captain of the Enterprise, as among the infected victims were Kirk's brother, sister-in-law, and nephew. Unfortunately, Kirk's brother, Sam, was found dead when Kirk arrived at the scene, and his sister-in-law died while under treatment on board the Enterprise.
After being infected by the creatures, Spock initially attempted take over the ship, trying to send it out of the system. Once Spock had used his Vulcan mental discipline to master his actions despite the pain, he was allowed to return to the surface to collect a specimen for study (the reasoning being that he was already infected, so no one else needed to be placed at risk).
The crew discovered useful information when a suicidal Denevan flew a shuttle into the Denevan sun to end his suffering. He was freed shortly before his ship was destroyed. Kirk, Spock, and Dr. McCoy were able to determine that high intensity light would destroy the creatures. The theory was tested on a specimen, killing it. Spock then volunteered to be placed in the test chamber to determine if he could be cured, as the Denevan was. The test succeeded, with the unfortunate side effect of Spock being blinded. Much to Dr. McCoy's regret, he soon later determined that only ultraviolet light was required, making that blindness unnecessary.
To destroy the parasites, the Enterprise deployed satellites to bombard the planet's surface with UV radiation, and they were used to exterminate the creature. Unlike with the case of the Silicon Entity, no one seemed to care that they were making an entire species extinct.
Dr. McCoy's claim that Spock would not have been blinded if he had been exposed only to UV radiation is incorrect. High-intensity UV radiation would be just as damaging to retinal tissue as broad-spectrum white light.
It is also unclear how UV radiation from satellites was able to penetrate into the darkened basements and other hiding places of the parasites.