Denevan neural parasite Denevan neural parasites were a dangerous form of life which had been migrating through the galaxy for at least two hundred years , inducing mass insanity on the planets they touched . Background to the encounter . A linear progression of insanity marked the spread of the parasites through space . According to 23rd century archaeologist s , 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 managed to destroy them in 2267 . Description . A Denevan neural 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 . Spock described a parasite as " resembling , more than anything , a gigantic brain cell . " Parasites were very resistant to phaser fire , even sustained and at high power levels . Method of attack . Denevan neural 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 . Exactly how the creature communicated its desires is unclear , but that it could do so is evident : Spock , while infected , attempted to land the " Enterprise " on Deneva , despite the fact that ( as he knew ) this was impossible . Later , he 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 . Eventually , this continuous painful stimulation led to insanity and death . Other data . Infected victims cooperated in working towards the parasites ' s objectives . This , and their enormous resistance to harm , led Spock to theorize that all of the parasites were parts of a single organism . How the parts communicated without a physical connection was not made clear . 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 . Vulnerability . 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 . Tests . The first attempt at a cure using this approach used exposure to a general spectrum of light at maximum intensity 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 Deneva 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 eyelid s , which instinctively closed to protect his eyes from strong light exposure , had prevented permanent damage to his optic nerve s . A constellation of 210 ultraviolet satellite s was placed in permanent orbit 72 miles around the planet . The light they generated reached all parts of Deneva , destroying the entire infestation . ( ) Information about the Denevan neural parasite was displayed by the computer in Keiko O ' Brien ' s school room on Deep Space 9 . ( ) A graphic of the neural parasite was also displayed in the infirmary aboard Deep Space 9 . ( )