The X-Files: The Host is a novella written by Les Martin. It was first published in 1997.
Description[]
A man's body is discovered in the New Jersey sewer system, and a "routine" murder investigation ensues. But when several workers are attacked by a vicious sewer-dwelling monster, it becomes clear that something much more sinister is going on underground. Mulder and Scully soon find themselves on the slimy trail of a creature as disgusting as it is dangerous.