The X-Files' central alien mythology arc often involved the mysterious black oil, and many fans see a connection between it and the episode "Ice." While it didn't factor into the revival, over the course of The X-Files' original nine-season run, the black oil was often an imminent threat, a sentient substance tied to aliens that seek to wipe out humanity and colonize Earth. Once infected with the black oil, it was rare to survive, or be restored to normal, outside of when a main character equipped with "plot armor" contracted it.
As usual for The X-Files, which quite clearly inspired Lost's formula of never answering a question without asking ten new questions, the black oil's origin and abilities weren't entirely spelled out. It's thought to trace back to before humans appeared on Earth, but obviously, no characters from back then are around to give firsthand accounts of that. What exactly the black oil does can also be a bit inconsistent on the show, especially whether it kills or infects.
While the black oil didn't officially appear until season 3, looking back at season 1's "Ice," some X-Files fans have noticed a striking similarity between the infectious substance and how the parasitic worm creatures in the episode operate. Are they connected? The truth might be out there.
The X-Files: How Ice Might Be Connected to the Black Oil
In "Ice," a clear X-Files homage to John Carpenter's The Thing, Mulder and Scully head to a remote Arctic research station to investigate unexplained goings on with the scientists working there, only to encounter a species of parasitic worm that gets under one's skin, literally. Like the black oil, the "Ice" worm creature is theorized to be of alien origin, landing on Earth via a meteor. Also like the black oil, the "Ice" worm is able to manipulate and control the behavior of its host, although not quite to the same extent. In "Ice," the worms' manipulation abilities are more about causing disorientation, paranoia, and confusion in its host, while the black oil completely takes over their will.
Most tellingly of all though, when a parasitic worm is forcibly extracted from its host, it squirts out a black, oil-like substance. Considering the black oil's ability to control life forms, the most likely possibility here is that the species of parasitic worm encountered the black oil at some point in the distant past, and was subsequently taken over by it fully. So basically, the worms were parasites before they met the black oil, but exposure to the black oil effectively enhanced them, making them more of a threat. Those involved with making The X-Files have yet to confirm or deny this possible connection.