The extraterrestrial virus-like substance in The X-Files is called black oil.
Also known as Purity, black oil is a sentient alien substance connected to the Colonists’ plan to colonize Earth. It usually appears as a dark, petroleum-like fluid and can enter a person’s body through the eyes, mouth, or other openings. Once inside, it may control the host or use the body to incubate a developing alien organism.
The substance is first prominently introduced in the season 3 episode “Piper Maru,” where it is found aboard a salvaged World War II submarine. Its mythology expands in The X-Files: Fight the Future and later episodes, revealing that it can survive in petroleum deposits and that the Syndicate sought to develop a vaccine against it. Russian researchers called it the “black cancer.”
Viewers sometimes confuse black oil with ordinary crude oil or with the alien colonists themselves. It is neither: within the story, it is an alien virus or life force with its own agency. The name “Purity” reflects the aliens’ terminology, while “black oil” is the common name used by characters and fans.