In the book, American soldiers refer to zombies as "Z's" and "Zack". Its a common practice for US military men to refer to things by their initials or the military equivalent (ie "T's or "Tangos" for terrorists). However, "Z" in the military alphabet is "Zulu". "Zack" is a reference to Zack Snyder, director of the 2004 remake of Dawn of the Dead, who author Max Brooks collaborated with for some of the DVD special features.