From Halopedia, the Halo wiki
Health in the Halo games is the player-character's physical well-being. In the games, a player's health is measured and displayed on the heads-up display (HUD). If a player's health runs out, their character dies; however, lowered health does not affect a player-character's stamina, movement speed, or other physical attributes.
Halo: Combat Evolved
Players in Halo: Combat Evolved have a limited, non-regenerating supply of health. Health can only be restored by picking up health packs. Health is shown on the HUD using a horizontal bar broken into ten segments. A player's health can only be reduced if their shields have failed.
In addition to being broken into segments, the health bar changes color to represent the approximate amount of health a player has left. The bar turns yellow when half or more of a player's health has been depleted; when this happens, the player should probably consider obtaining a health pack soon, though the need is not urgent. When only two segments remain on the bar, it will turn red, indicating that the player should find a health pack as soon as possible, and should avoid dangerous situations until then. When only one segment remains, the sound of a beating heart can be heard.
Contrary to popular belief, there is a small amount of health in Halo 2, though it is not explicitly shown on the HUD. Health recharges with a player's shields; this is canonically explained by the presence of biofoam injectors in the MJOLNIR armor that the Master Chief uses in this game. The shield bar appears to change color slightly to reflect the player's health.
The health system in Halo 3 is a bit more complicated and, like Halo 2, has no visible health bar. Bungie has stated that players have 115 hit points: 45 health points and 70 shield points. When a player's shields fail, their health becomes vulnerable. If the player avoids taking damage for 10 seconds, their health will slowly regenerate at a rate of 9 points per second. Health and shields are independent; a player's shields can regenerate without their health following suit, so full shields do not necessarily mean that a player is safe from death.
Halo 3: ODST
In Halo 3: ODST, players once again have a health bar that is shown on the HUD. A player's health in ODST can only be damaged after their stamina is depleted. Because players play as ODSTs (who are weaker than armored Spartans), health in this game is depleted much faster than in previous games of the series. As in Halo: Combat Evolved, health can only be restored using health packs.
In Halo: Reach, Spartans have limited regeneration and a visible health bar. When a Spartan is damaged, after four seconds, his/her health regenerates to the nearest third. If a Spartan is brought down to or below 2/3 health, the remaining health can only be restored by picking up a health pack, or, by using a Drop shield.
Elites cannot use health packs, but their health can regenerate completely, though more slowly than a Spartan's regeneration.
In Halo 4, Spartans regenerate very similar to how it was done in Halo 3 and have no visible health bar.
Halo: Spartan Assault
In Halo: Spartan Assault, Spartans regenerate health very similarly to how it is done in Halo 4.
Halo: Spartan Strike
In Halo: Spartan Strike, Spartans regenerate health exactly the same as how it is done in Halo: Spartan Assault.
Halo 5: Guardians
In Halo 5: Guardians, Spartans regenerate health very similarly to how it is done in Halo 4 with the exception of a visible health bar on the HUD. Should the player's health reach 0.3% or lower, the health bar will then be seen flashing red. If the player plays as John-117, Frederic-104, Kelly-087, or Linda-058, this health bar will be seen flashing red twice as quickly. A flashing red health bar is most easily achievable in Forge by creating and activating a script that damages players equal to 99.7% of their maximum health and then entering said script's trigger area.
In the campaign, in most cases, if a character's health does reach 0%, he/she will not immediately die, but will instead be incapacitated and must be revived within 15 seconds (longer for AI-controlled teammates). If damaged further by weapons (especially explosives, but usually only for AI-controlled Spartans), not revived in time, or if all other Spartans on the fireteam are dead or incapacitated, only then will that character (and all other incapacitated characters) actually die, which also usually results in dialogue from one of the surviving characters saying to hold together (e.g. Fred saying "We lost Chief! Hold position!" if Chief dies, or Buck saying "Tanaka's out! Regroup and hold." if Tanaka dies). However, the dialogue does not occur if a character commits suicide by falling off the map or killing himself/herself with weapons.
The following can also cause a character to instantly die and become unrevivable until they respawn:
Player's health can be fully replenished by obtaining a single health pack; this does not regenerate shields, however. Health packs are only featured in Halo: Combat Evolved, Halo 3: ODST and Halo: Reach.
In the campaign and multiplayer modes of Halo: Combat Evolved, Halo 3: ODST, Halo: Reach, Halo 4, and Halo 5: Guardians (in the campaign, most often happens to AI-controlled teammates), players can take damage if they fall from too great a height. Such fall damage can kill players who fall from great heights, although fall damage is reduced in Halo 5: Guardians when compared to previous installments, also with regards to the height required for damage to occur. Fall damage is not present in either Halo 2's or Halo 3's campaign or multiplayer, however, though falling deaths can still result from contact with a kill barrier. Fall damage has a fixed amount of damage, dependent on how far the player falls regardless of any difficulty. Fall speed is also a contributing factor. The faster the player falls, the more health is lost regardless of the height from which the player falls. Rarely, it is possible to prevent fall damage by using certain weapons such as the Gravity Hammer and with the correct timing, negating or decreasing the fall damage.