Tarchannen III species The Tarchannen III species were a non - sentient species native to the planet Tarchannen III . They had a characteristic mimetic ability that made them nearly invisible . A radiant reaction in the skin simulated light and acted as a kind of biological cloaking device . This , coupled with their low body temperature , made them nearly impossible to detect , although the effect of their skin could be overcome by ultraviolet light . Reproduction in the species was accomplished by embedding a parasite into the thymus of a humanoid host . The parasite then used the host ' s immune system to spread genetic instructions , transforming the DNA of the host to match its own . This virus produced with weak cell walls , and contact with the cells themselves was not contagious . Depending on the host species , the parasite could lay dormant for five year s before these changes started . In 2362 , the 49 - member crew of the Starfleet outpost on Tarchannen III was infected and underwent the genetic changes . With no word of the event to Starfleet , the was dispatched to investigate the crew ' s apparent disappearance , but found no trace of the missing personnel . In 2367 , the members of the " Victory " away team that had investigated the disappearance began to desert their post and return to Tarchannen III . It was discovered that these crewmen , including Geordi La Forge , were infected by the parasite and transforming into the Tarchannen III species . In the case of Lieutenant Commander Susanna Leijten , as observed by Doctor Beverly Crusher ( CMO , ) , the parasite had remained dormant for five year s before beginning to transform Leijten ' s DNA . The patient first began to exhibit symptoms of anxiety . Tests showed that her blood chemistry was abnormal and that she had a histamine response to something unknown . Dr . Crusher then performed a complete blood and tissue analysis , but the tests were inconclusive . Leijten began to suffer tremors in her hands , and increased visual sensitivity . Shortly after this she collapsed and it was discovered that she had blue vein - like growths on her neck and that two of the digit s of her right hand had grown into one . The patient ' s physical form continued to rapidly change . The growths soon covered her entire body and her eye s changed color . She developed a strong sensitivity to light and her blood pressure dropped . Dr . Crusher administered a T - cell booster to attempt to stabilize her immune system . An examination of a cell taken from her skin showed it to be identical to alien cells found during an earlier investigation of the missing crewmen . As Susana was not herself contagious , Dr . Crusher was able to continue further study . She discovered the location of the parasite in Leijten ' s thymus , as well as the nature of its operation . She was able to remove the parasite and regenerate the DNA of the patient . Dr . Crusher was able to successfully replicate this procedure with Geordi La Forge ; however , for Emilita Mendez and Anthony Brevelle , there was no hope . ( ) Background information . According to an internal reference document , listing all the aliens seen in , which was circulated around the time of the production of TNG Season 5 , this species was identified as " Tarchannes " . They were further given the description of " non - sentient humanoid race , frightened by normal light and visible only under ultra - violet light " .