Flood's core gameplay is identical to Infection's core gameplay: players are split into a Survivor team and a Flood team, and survivors must avoid being killed and converted to the Flood team. The principal difference between Flood and Infection is the addition of a new player model for the Flood: an infected Spartan's body was stripped of its hard armor plating and covered in a Flood carapace. Altered settings also gave Flood players a special armor effect that trailed behind them and a first-person screen effect that distorted their vision.
Flood players did not spawn with weapons and couldn't pick any weapons up; they attacked using a fleshy claw that behaved similarly to the Energy Swords used in Infection. Flood players were also incapable of sprinting.
Flood's default settings dictated that two players would start on the Flood team, and rounds would last for three minutes. Unlike Infection in Halo: Reach, infected Spartans in Halo 4 are equipped with energy shields, making it harder to kill them with headshots. Survivors would spawn with shotguns and magnums, and both teams had three loadouts to pick from, each of which offered a different armor ability.
A new public variant called Hivemind was added on July 1, 2013. In Hivemind, Spartans appeared in their chosen colors, instead of the default green. Survivors start with Assault Rifles and Magnums, and are able to pick up other weapons around the map, including Shotguns. It takes two hits from a Flood to kill a survivor. Neither side has Armor Abilities.
Notably, several Custom Game options were missing from Flood. Flood Player Traits specifically omitted the options for starting weapons and weapon pickup, even though there were no technical limitations preventing the Flood character model from using weapons. Flood Loadouts were no longer visible in-game as they still relied on the Elite Loadouts options, which were totally removed from the Custom Games user interface. Both the survivors and the Flood had forced colors (green and grey, respectively) which could not be changed, as the Forced Color option was totally removed from the Custom Games user interface. The option to control players' access to sprint was never made available in the Custom Games user interface. The "Weapons on Map" setting was also hidden from the user interface but retained within the game, and it was forced to "off" for Flood. As such, until Hivemind launched, it was impossible to have a Flood variant with weapons on the map, and even after Hivemind launched, it was impossible to configure any of the weapons or equipment used by Flood players, and it was impossible to fully configure or balance Flood movement settings. The Safe Havens settings introduced in Reach were also removed and were never re-implemented.
The gametype features exclusive medals that can only be earned by playing this particular game mode.
- Flood Conversion - Convert a Spartan to the Flood.
- Alpha Conversion - Convert a Spartan to the Flood as an Alpha.
- Infector - Kill 2 Spartans in a row as a Flood without dying.
- Carrier - Kill 3 Spartans in a row as a Flood without dying.
- Juggernaut - Kill 4 Spartans in a row as a Flood without dying.
- Gravemind - Kill All Spartans in a row as a Flood without dying.
- Flood Kill - Kill a Flood.
- Flood Kill Assist - Assist killing a Flood.
- Flood Hunter - Kill 4 Flood in a row as a Spartan without dying.
- Flood Survivor - Kill 6 Flood in a row as a Spartan without dying.
- Flood Exterminator - Kill 10 Flood in a row as a Spartan without dying.
- Last Man Standing - Be the last Spartan survivor.
- Final Conversion - Kill the last remaining Spartan.
- Flood Victory - Contribute to the Flood total conversion of all Spartans.
- Ancient One - Survive the entire round as a Flood and convert at least one Spartan.
- Clever - Survive the entire round as a Spartan.
- Flood had a networking glitch in which a Flood player would sometimes appear without Flood traits. For example, they would retain their chosen armor color instead of having a forced color fitting either team.
- Flood had a scripting mistake that caused Alpha Flood to revert to standard Flood after their first death. This was never fixed.
- Flood retained the Reach-era Infection glitch that affected game variants with limited lives.
- When playing as the Flood, the most viable strategy is to pick Thruster Pack, launch oneself at survivors at high speed, and use a melee lunge to divebomb directly at them and kill them. This strategy was the most effective means of killing survivors entrenched in imbalanced map locations.
- When playing as a survivor, Active Camo was the best loadout to use. Flood players could not perform melee lunges against survivors with Camo active, rendering the most viable Flood strategy totally worthless.
- Official Xbox Magazine ,October 2012
- Halo Waypoint: The Halo Bulletin 10.3.12