Call of Duty Zombies, also simply known as Zombies, is a recurring game mode in the Call of Duty series of first-person shooter video games published by Activision. Players in the mode are put up against an endless horde of zombies, and are forced to survive for as long as possible. Killing zombies grants the players points, which can be used to obtain more powerful weapons and unlock new areas of the map. The waves of zombies progressively increase in difficulty, becoming faster and having increased health. Additional features available to the player in most versions of the mode include "Perk-a-Colas", which can upgrade players abilities, and a "Pack-a-Punch" machine that allows players to upgrade their weapons. Many maps feature easter eggs and quests, which players can interact with and complete. These range from quests that trigger secret songs to those that progress the overarching storyline of the Zombies mode.