Black oil, or black cancer is the name given on The X-Files to a form of extraterrestrial virus with the ability to control the host. In the series, the black oil (Purity) represents a form of E.B.E and an invasion force. It appears as a liquid with the consistency of crude oil, can move on its own, and is sentient. It can hibernate for thousands of years, usually in petroleum deposits. When it comes into contact with humans, it absorbs itself into the host, sometimes appearing to move directly under the skin and over the eyes of the victim. After pooling in the third ventricle of the brain (Vienen), it takes possession of the person and controls them. The oil is able to kill other humans by inflicting massive doses of radiation, though it is somehow able to simultaneously protect its host from injury. According to a conspiracy theorist Fox Mulder encounters, the Black Oil was used by Iraq during the Gulf War.
The Syndicate, the shadow government of The X-Files, in cooperation with the alien colonists, develop a way to quietly introduce the virus into an unsuspecting public, through the use of bees. The colonists would then be able to use human beings as a slave race. The Syndicate, however, secretly try to create a vaccine to protect themselves, which they codename "Purity Control." While the Purity Control project ultimately fails, a rival Russian shadow group is successful in developing a weak vaccine which eventually falls into the hands of the Syndicate.
Later in the X-Files film, it is discovered that the Black Oil is able to use a human body as an incubator in order to gestate a gray alien being. The Syndicate is dismayed to learn this, but pretend to work alongside the colonists while attempting to strengthen the weak Russian vaccine. Because human-alien hybrids are immune to the virus, many of the conspirators have allowed the abduction of family members to acquire this immunity and thus be safer in the future when colonization begins.
According to the Well-Manicured Man, the entity first walked the earth before dinosaurs. Then, as told by Cigarette Smoking Man, the oil influenced human evolution and eventually went into hibernation underground. Cassandra Spender recounts that the colonists are planning to use the oil as part of their plan to repopulate Earth as they have done before to other civilizations in the universe.
The plot to cooperate with the alien colonization plan is implemented with the aim of being given access to the Black Oil, in order to try and develop a vaccine. This attempt is semi-successful, as the "weak vaccine" administered to Scully while in the Antarctic alien ship causes the entire ship to depart its underground residence.
The Black Oil was first introduced in the season three episode "Piper Maru". It is believed that the basic concept behind the Black Oil was taken from an earlier episode, "Ice", which features a parasite that similarly took control of its host; in this episode, when the parasitic worm is forcibly removed from a host, it "bleeds" (or possibly vomits) black droplets.