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 USS Victory 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 Tarchanne.
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 cells 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 years before these changes started.
In the case of Lieutenant Commander Susanna Leijten, as observed by Doctor Beverly Crusher (CMO, USS Enterprise-D), the parasite had remained dormant for five years 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 digits 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 eyes 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 Leijten 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 Mendez and Anthony Brevelle, there was no hope. (TNG: "Identity Crisis")
The name of this species was not mentioned in dialogue, but comes from an internal reference document written around TNG Season 5, later released by Roddenberry Entertainment.  In it, they were described as "non-sentient humanoid race, frightened by normal light and visible only under ultra-violet light". They were otherwise identified in the episode's script as "TARCHANNEN CREATURES".
According to the entry for the "Ultraviolet Light Beings of Tarchannen III" that appeared in Star Trek: Aliens & Artifacts (pp. 115-116):
This was a gradual make-up, showing Geordi's mutation into a reptile until the final scene where he is completely changed and wants to remain on the planet. His makeup, a great collaboration between the wardrobe and make-up departments, consisted of latex bicycle pants, with hands and feet appliances, facial appliances, special contact lenses, and veins that six artists glued on by hand.
In the first stages of Geordi's transformation from human to ultraviolet reptile, little blue veins begin to appear on Geordi's neck, and like little streams of water coming together they start to connect. Each vein was a separate minuscule appliance that had to be glued on by hand. The more advanced the parasitic disease became, the more veins had to be glued on and connected. The same appliances had to be glued to Geordi's hands and feet at later points in the shooting of the episode to show the progress of the disease.
At the same time as the veins continued to proliferate, Geordi had to appear increasingly lizard-like, so at an intermediate stage of the transformation he was fitted with a nose appliance that looked like a lizard's nose. The final transformation makeup took over six hours to apply, because it consisted of full appliances to LeVar's head, hands, and feet, and latex pants onto which the makeup artists glued each vein and then hand-painted them with a fluorescent pigment to make them glow under ultraviolet light. For a final touch, LeVar was fitted with special yellow contact lenses, prepared for him by Dr. Morton Greenspoon. The doctor's assistant then put drops in LeVar's eyes– the same ones that are used to test for glaucoma– that made the eyes glow when exposed to black light."
According to Michael Westmore, it was the longest make-up ever done on The Next Generation. ("Departmental Briefing, Year Four: Make Up", TNG Season 4 DVD special feature)
A Playmates action figure of Geordi as this species that was identified him as a "Tarchannen III Alien".