There seem to be an infinite amount of them, and in all levels they continually increase in HP as the game goes on, eventually reaching a point where the only feasible way to kill them is with weapons that are automatically one-hit kills. There is no way to win zombie mode, the only goal is to survive as long as possible before eventually dying, although most of the later maps in Black Ops contain complex easter egg missions with unspoken objectives. In the earlier Zombie maps from WaW the zombies weren't as "zombiefied" and still resembled humans, but in the Black Ops maps they become progressively more corpselike and mutilated.
Call of Duty: World at War
They first appeared in World at War for Xbox/PS3/PC in a simple easter egg minigame intended as a joke. When the minigame unexpectedly became extremely popular, the developed Treyarch was prompted to make more zombie levels as DLC. The third zombie level, "Shi No Numa" from the second DLC pack, was the first to flesh out Nazi Zombies as more than a simple easter egg. Instead of Nazis, it featured Japanese zombies, as well as the first special variety of enemy in the form of Hellhounds, and introduced four playable characters with distinct personalities: American hero Tank Dempsey, zealous Japanese soldier Takeo Masaki, Russian Nikolai Belinski, and the sociopathic Nazi Dr. Edward Richtofen, who were all forced to work together to survive the zombies. These characters became the main protagonists of the Zombies series, and were the playable characters in all maps that followed except 3, one of which they made an appearance in as NPCs. Each new map establishes that they were eventually brought to the new location by a random malfunction of the teleporters that appear in all levels.
The fourth and final Zombie level in WaW, Der Riese, introduced few new gameplay elements but did establish an actual serious storyline. Hidden throughout the level, which takes place at the German factory where the zombies were supposedly created, are various hidden messages and clues which reveal the disturbing origin of the zombies. Der Riese was the location of a top-secret Nazi experimental project called "Group 935", headed by a scientist named Dr. Ludvig Maxis. It appears that the main focus of the project was a teleportation device powered by Element 115. The first several test runs of the device resulted in the deaths of the subjects, but it was later discovered that their bodies were reanimated by Element 115. When he discovered this, Dr. Maxis began trying to create an undead army, but could not find out how to control the zombies and eventually began running out of funding for the experiments. Presumably due to a lack of human test subjects, he sent his daughter Samantha's dog "Fluffy" through the teleporter and the result was the first Hellhound. Before they could leave, Richtofen, who was Dr. Maxis' assistant, betrayed him and locked them in the room with the dog, which killed Samantha, before escaping through the teleporter to locate more Element 115 and create his own zombie army. Dr. Maxis later committed suicide when zombies overran the Der Riese facility.
Call of Duty: Black Ops
Zombies Mode returned in Call of Duty: Black Ops two years later, which directly continues the story arc from the previous zombie levels. The original release of the game includes "Kino Der Toten" ("Theater of the Dead"), which again has Dempsey, Takeo, Nikolai, Richtofen and Nazi zombies, as well as "Five", a very bizarre level where the zombies attack the Pentagon and you play as John F. Kennedy, Richard Nixon, Fidel Castro and Robert Mcnamara, and "Dead Ops Arcade", which is an overhead-view arcade-style shooter with totally different gameplay than all the other zombie levels. These levels introduce a third enemy type, the Crawler Zombies, hideously deformed creatures that move on all fours and emit Nova 6 gas. "Five" features a special "boss" enemy known as the Pentagon Thief, who appears only in special rounds and although he can't actually kill the players, he takes away your weapons and is very hard to kill. "Dead Ops Arcade" has a giant zombie gorilla boss called the Cosmic Silverback.
The four DLC packs for Black Ops introduced "Ascension", "Call of the Dead", "Shangri-La" and finally "Moon". "Call of the Dead" is unique in that instead of the four usual characters, it stars a group of real-life celebrities who are trying to make a new horror film when the zombies attack. The playable characters are Sarah Michelle Gellar, Danny Trejo, Michael Rooker and Robert Englund. George A. Romero, who was directing the film they were trying to make, has become a zombie himself and is a new boss. Hidden in this level are the original four characters, who have been teleported behind a door and are trapped, and the player(s) can rescue them and let them move on to the next map, Shangri-La.
"Moon" is the final level in the "Black Ops Zombies" saga. Black Ops II introduced Green Run, a map taking place after Richtofen's Grand Scheme and in Washington State. The characters are Marlton Johnson, Abigail "Misty" Briarton, Russman, and Samuel J. Stuhlinger. There are 3 modes, "Survival", "Tranzit" (only "Bus Depot"), and "Grief". The "Revolution" DLC Pack introduced Chinese zombies in Shanghai called "Die Rise" and a new mode called "Turned". Minions were introduced and they are similar to crawlers from Black Ops I in "Die Rise"
In "Uprising", a new map for "Zombies"' is "Mob of the Dead". It has 4 new characters, Albert "Weasel" Arlington, Billy Handsome, Salvatore "Sal" DeLuca, and Michael "Finn" O'Leary. It has a new boss character called Brutus, a heavily armed guard zombie. It takes place at the Alcatraz and Golden Gate Bridge in San Francisco during the 1930's. It also introduces "Cell Block" a new "Grief" location. Until the next DLC, "Mob of the Dead" is the final map in the "Zombies" Saga.
Call of Duty: Black Ops III
Zombie makes another return in Black Ops III