Modern Warfare 2's single-player campaign ended up being a direct continuation of Call of Duty 4: Modern Warfare's realistic storyline. According to MW2 scriptwriter Jesse Stern, there were a number of other ideas on the table for the game's plot.

"In the beginning we talked about having things in Modern Warfare 2 like outbreaks, viruses, chemical warfare, and even outlandish things such as aliens and the living dead," Stern told GamePro. "We worked our way through it before we finally ended up with a more grounded version of the real world as it's depicted in Call of Duty: Modern Warfare 2."

The other Call of Duty developer, Treyarch, put a zombie multiplayer mode in last year's Call of Duty: World at War. However, they didn't include any supernatural elements in the actual campaign. It seems like there are more than enough games out there with alien and undead antagonists, no?