Apparently because of poor box-office results, this film was the last to be released by Film Ventures International (FVI). Edward L. Montoro, president of FVI, disappeared shortly afterward, taking $1,000,000 from the company's coffers with him. Nothing has been heard from him since.
A sequel was written in the early 2000s but was never filmed. Bo Hopkins was attached to the project and was set to reprise his role as Sheriff Will Stewart. A part was also specially written for Ken Foree.
According to writer John C. Kruize, the original story (while under the title "The Pestilence") "concerned secretive and illegal germ warfare research being conducted by the U.S. military in a remote area of the Rocky Mountains. A lab worker is infected by some type of mutant flu virus and carries the contagion to an isolated town." He admits Stephen King's novel "The Stand" was an influence.
After the script was bought, the writers were contractually obligated to deliver a complete rewrite of the script within 30 days. They actually delivered it in 2 weeks by taking time off from work, working late, and working weekends.
President of Film Ventures International Edward L. Montoro was responsible for most of the changes in the script after the rewrite process. Writer John C. Kruize admits some of the changes were good or acceptable (changing the germ warfare premises to toxic waste poisoning) but most weren't. During this process the title was changed to "Night Shadows", which the writers thought was weak.
Some DVD companies mistakenly and confusingly used the poster artwork of Forbidden World (1982) as the cover art for this film, because both films were released under the title "Mutant" in various countries.
Writers John C. Kruize and Michael Jones decided to collaborate on writing a horror script in 1980 while working in the mail room at MGM Pictures. They wrote the script for Night Shadows (which was titled "The Pestilence" at the time) while on their lunch breaks and after work. They finished the script in only a few months.