Zombies will be the primary enemies of survivor characters in ZDay. Though in most works of fiction, zombies will either walk or run, ZDay will feature a variety of zombies, with different abilities. Also, unlike most previous survival horror games, player characters can actually turn in to zombies when they become infected, adding another zombie to the horde.
The zombie virus of ZDay was created by an as of yet unnamed government as a biological weapon. It was contained in a top secret underground facility, where it was being tested on rats in an electronically quarantined environment. Unfortunately, on December 21st, 2012, there was a freak power outage- the sealed doors swung open due to a faulty fail-safe system, and the plague-harboring rats were freed. The virus spread quickly- in just four days, it was a pandemic of global proportions, throwing the world in to chaos.
Though little is known about the combat mechanics of ZDay, it should be safe to assume that zombies will be felled by gunfire, possibly limited only to the head. Some zombies will be more dangerous than others, as freshly turned infected will be able to run, while more decayed zeds will be limited to shambling.
There will possibly be a multitude of zombie types in addition to fast and slow human infected, with plans for infected rats, dogs, and possibly birds. As for special zombie types (such as the Special infected encountered in Left 4 Dead) there is as of yet no information.