Zombie are a scourge to be eliminated at all cost, and should be treated as such. Maybe he could write a story about the passenger virus and how it came to be in the story line and about what the planetary union and other factions did to combat it. Maybe it could have something to do with the wasplike creatures, or the aliens the tank crew get eaten by in crypt, or maybe it's a separate sentient being on it's own. I think that would be an awesome addition to the storyline.