As with most first person shooter games, the player has a health system in the Call of Duty series.
In all games before Call of Duty 2, the player needed to pick up health kits in order to regain health. Starting with Call of Duty 2, health kits were removed in favor for a regenerative health system. Whenever the player is below 33% of their maximum health, tunnel vision or a red tinge (depending on the game) will appear on screen, accompanied by a raised heartbeat and strained breathing.
Starting with Call of Duty: Modern Warfare 2, blood replaces the tunnel vision effect and gradually appears as the player takes damage. Assuming the player isn't taking damage or dead, health regenerates completely within five seconds of being injured. A player can tell when their character has successfully regenerated their health by an exhaling sound. In Hardcore, health regeneration is disabled, and maximum health is reduced to 30 points. In Old School, health regeneration is also disabled, and the player's maximum health is increased to 200. In Zombies, purchasing the perk Juggernog increases health to 200.
In Call of Duty: Modern Warfare 3, when using both the Assault Juggernaut and Juggernaut Recon, a player will only take 8% of all normal damage. Essentially, this means the player will have 1250 health points while in a Juggernaut suit.