Mulder and Scully investigate a body found in sewer after being reassigned to different departments.
The Host is the second episode of the second season written by Chris Carter, directed by Daniel Sackheim. It's unconnected to the series' wider mythology and is a standalone episode (a 'Monster-of- the- Week' story) that said, it does develop Mulder and Scully's relationships and characters effectively.
David Duchovny gives a physical performance as bizarre creepy fluke-like man goes on a rampage in the sewers of New Jersey. The atmosphere is pretty grim. The bite make up is well executed. There's plenty of blood and gross moments notably where a man eats a tube of toothpaste, a worm-like creature appears from the body at an autopsy (performed by Scully) and where a man spews out the slimy fluke creature.
With long dark tunnels, moment of men being attacked and dragged under the sewerage water, if horror is your thing this is one of the stand out episodes.