Purity, more commonly referred to as black oil, and called "the black cancer" by the Russians, was an alien virus that thrived in petroleum deposits underground on Earth. The virus was capable of entering humanoids and assuming control of their bodies. It was sentient and was capable of communicating. It is the "life force" of the alien colonists, which they seemingly use to reproduce their kind, as well as infect other alien races in order to conquer the universe.
According to the Well-Manicured Man, black oil was the original inhabitant of the planet Earth.
In 35,000 B.C., two primitive humans pursued a set of strange tracks through the frozen wasteland that was North Texas. They followed the tracks into a cave, where they were attacked by a long-clawed alien that had left the tracks. Although one of the men was killed, the other managed to injure the creature. As the alien lay dormant on a rock, a form of black oil poured out of its body, covered the remaining man and painfully infected him.
In 1945, a squadron of P-51 Mustang airplanes escorted a B-29 transporting a nuclear bomb similar to those used on Japan during World War II. However, the B-29 and its accompanying squadron of aircraft crashed in the Pacific Ocean after they had a hostile encounter with a UFO that also crashed.
Later the same year, an American submarine named the Zeus Faber was deployed in an attempt to locate the crash site. The Zeus Faber found the sunken squadron but the submarine's crew also unknowingly brought the black oil aboard. The alien substance possessed the Zeus Faber's commanding officer, Captain Kyle Sanford, and manipulated him into refusing to return to port without providing an explanation.
When the crew turned on the possessed captain and rendered him unconscious (or possibly killed him), the alien entity left his body and then seemingly the submarine itself, returning to the ocean. The Zeus Faber's crew complement had been 144 when it left port, but only seven survived the return journey to Pearl Harbor. The majority of the crew had died from radiation burns due to exposure to the possessed Captain Sanford, though none of the crew were ever given an explanation for the awful burns and the survivors never discovered a reason for their own survival. The incident was never revealed to the public.
In 1995, the UFO that had crashed with the squadron of US airplanes was recovered by an American salvage ship named the Talapus. (TXF: "Nisei") While the UFO was secretly taken to San Diego and secured within a guarded warehouse there, the black oil remained within the Pacific Ocean.
In 1996, the French salvage ship Piper Maru was on a mission to locate the sunken squadron of airplanes when a diver, Bernard Gauthier, discovered one of the planes. The Piper Maru's crew had been monitoring Gauthier's progress when they lost all contact with him. Gauthier continued to inspect the plane and found that a pilot was bizarrely still alive inside, trapped in the cockpit and possessed by the black oil. By the time Gauthier returned to the surface of the water and was recovered by the Piper Maru, he had also been possessed by the alien substance. Gauthier claimed that he was unharmed and asked his crew mates to help him remove his diving suit. One man found a thin layer of oil that covered the diving suit but thought nothing sinister of it. Like the men aboard the Zeus Faber, the black oil also exposed the crew of the Piper Maru to extremely high levels of radiation. The ship slowly journeyed to San Diego, steered home by the possessed Gauthier - the only man not affected by the radiation.
The French government classified the nature of the contaminant that had irradiated the Piper Maru's crew and refused to release that information. In fact, the US Navy inspected the Piper Maru but found nothing unusual nor any radiation aboard the vessel.
The crew members suffering from the radiation were admitted to San Diego Naval Hospital but determining a course of treatment for their deteriorating condition was complicated by the secrecy regarding the incident. What could be determined was that the men had been exposed to an unnaturally high level of radiation, with a high rate of absorption. Symptoms included spontaneous internal bleeding in the mouths and intestinal tracts, blood in the urine and severe delirium — the pre-advanced stages of a coma. Though Gauthier had been held at the hospital for one day, he seemed to be in very good health and discharged himself the next morning. Joan Gauthier infected with the black oil. Gauthier, controlled by the black oil, returned to his home in San Francisco, California, where he lived with his wife, Joan.
In late 1996, a diplomatic envoy from Russia landing in Hawaii was intercepted by security guards and searched for being belligerent with them. During the search, the guards discovered a canister containing what the envoy described as a bio-hazardous material. The guard dropped the canister, shattering it and releasing its contents: Purity.
Another envoy carrying a sample of the same rock from Tunguska, Russia, arrived at Dulles International Airport in Washington, D.C.. With the ironic assistance of Alex Krycek, Agents Mulder and Scully attempted to intercept the courier. However, he fled but dropped his diplomatic pouch that contained the mysterious rock. Krycek knew that the Syndicate would be looking for the rock, and he was eager to take revenge against the Cigarette Smoking Man. Krycek was then kept in Walter Skinner's apartment while Mulder and Scully tried to learn more about the rock. The CSM approached Skinner the following day demanding to know the whereabouts of the pouch, but Skinner was unable to answer because the agents hadn't revealed it to him. Scully took the rock to the NASA Goddard Space Flight Center to have it analyzed with Dr. Sacks. Sacks discovered that the rock was the same as meteorites, which prompted Mulder to travel to Tunguska in search of answers.
Mulder and Krycek traveled to Russia, where they discovered prisoners of a gulag mining for extraterrestrial rocks like the one they had intercepted. However, they were discovered and imprisoned. Betrayed by Krycek yet again, Mulder was then subjected to an experiment wherein he was exposed to the black oil. However, he survived unscathed because the Russians had developed an effective vaccine. Meanwhile, the Russians discovered the outbreak in the United States. Vassily Peskow was sent to take care of it. Ultimately, Peskow successfully covered up the presence of the black oil and destroyed much of it by blowing up an oil well in North Dakota.
In 1997, Alex Krycek discovered a boy who witnessed the burning of a group of abductees by the faceless alien rebels. In order to ensure that no one else found out what happened, Krycek infected the boy with the black oil and brought him to the United States, to barter him with the Syndicate. He also managed to smuggle a sample of the vaccine that the Russians developed, which he gave to the Syndicate. They tested it successfully on Marita Covarrubias, who had been infected with the virus by the boy.
In 1998, several people were infected with a strain of the virus lying dormant beneath suburban Dallas, Texas. The Syndicate, fearing an outbreak, hastily tried to contain the virus, and learned that it not only inhabits and controls its host, but also uses it as an incubator for a new alien life-form. Agents Mulder and Scully investigated the deaths of the infected people, and were led to a site where genetically-modified corn pollen was being harvested in order to carry the virus, which will be delivered by bees during the alien invasion. Scully is later stung by one of the bees, and infected with the virus. The Syndicate abducted her and took her to a research facility in Antarctica, where she is incarcerated along with hundreds of other infected hosts. Mulder pursued her and was able to administer a vaccine to her, which had been provided by The Well-Manicured Man.
In 2001, the black oil was encountered again. The Galpex oil company ran an oil rig in the Gulf of Mexico that tapped a petroleum reserve containing the virus, infecting the entire crew. Simon de la Cruz and Diego Garza were remarkably immune to the virus. Simon was killed by Bo Taylor after he attempted to contact the mainland and Garza went into hiding aboard the rig. Fox Mulder, who had recently reappeared following his abduction, brought the FBI into the situation after Simon's corpse washed ashore in Texas. Furious that Mulder had gotten the Bureau involved, Alvin Kersh sent John Doggett to clear everything up. However, Mulder disobeyed orders and traveled to the rig on his own, beating Doggett there. With help from Scully back in Washington, Mulder was able to confirm that the black oil was indeed present aboard the rig. Doggett was nearly killed by Taylor as well and was able to witness the virus for himself. As Mulder and Doggett attempted to contact the mainland, the infected crew members tried to kill them - but they stopped as soon as Mulder destroyed the radio. Almost at once, the crew then tried to blow up the platform, but the agents were rescued before it went up in flames completely. Although Mulder took the fall for the destruction of the platform, he exhorted Doggett to keep Galpex from continuing operations on the virus-infected oil reserve.