The Zombies have varying speeds, They are:
The zombies seem to have reverted to a primal cognitive state, as they will attempt to kill or incapacitate any human through melee attacks. They are unable, or possibly unwilling, to use any weapons, and might dodge bullets but will not avoid grenades. Even when both legs are blown off, the zombies will continue to crawl and attempt to kill any living human. The zombies also show an odd behavioral characteristic in the sense that they will not attack a human being that is downed, yet instead go for the humans still alive. It is unknown why this is done, but it may be a thought process in what's left of their primal mind that the humans that are still healthy pose them a threat. It is also thought that since the player is on the ground, the zombie thinks that the player is dead. As shown by the Der Riese radio messages, the zombies were meant to be controlled, but they would always end up going berserk.
Some zombies will die from only a few rounds from an M1911, while other zombies can withstand up to or exceed two blasts from a Ray Gun. This depends on the round as the zombies have more health in later rounds. At round one zombies start with slightly more than 100 health which increases at a constant rate until it reaches 950 at round 9. After round 9 zombie health is multiplied by 1.1 each round as it approaches ~5.5 million at round 100. Zombies also have varying movement speeds, from a slow stumble to an outright dash. In Verrückt and in Black Ops, the zombies seem more evolved, being able to reach through boarded windows and swipe at the character, and can hop on and off of window sills. It can also be noted that in Verrückt, some zombies march to the windows, as if marching with an army. Finally, (again in Verrückt) the Zombies seem to have evolved enough to attain the ability of speech, or at least, something that can be considered speech. The zombies are often heard shouting, "Sam!" and scream, laugh and yell more than just moan like in Nacht Der Untoten Zombies that are seen marching and some with brighter Nazi bands on their right arm (which sometimes is covered by a rolled up sleeve) are usually found to be carrying power-ups, such as Insta-Kill, Nuke, Carpenter, Fire sale (Black ops only), and Max Ammo. The zombies also look different from each other. Some look as if they were burned to death, some look intimidating, while some are missing arms, or even have exposed bloody rib-cages. On Verrückt zombies sometimes appear to have staples inserted into their head, giving the assumption they died during brain surgery. Most of the zombies in black ops have decomposed faces and have a slight change in their speed than other zombies. On Ascension, the zombies faces seem to have been warped more than zombies before, their flesh sagging in many places on their face. Also, it may not have been noticeable on the levels where the zombies are Nazis due to dark clothes, but on Ascension where some zombies are wearing white lab coats, they are covered in smears of blood.