Cold-Blooded

Cold-Blooded is a Tier 2 perk featured in Call of Duty: Modern Warfare 2's multiplayer. The perk is unlocked at level 25.

Overview
Cold-Blooded's main usage is for the avoidance of killstreak rewards - it allows the user to be invisible to all forms of air support. The vision for Chopper Gunners, AC-130s and Predator Missiles show players with the perk active, but do not pinpoint them with a red outline square. Cold-Blooded users also can be killed by collateral damage from other killstreaks, for instance standing between a user of a different tier 2 perk while a Sentry Gun is firing upon them. Users of the perk also have no white thermal silhouette while being viewed in Thermal Scopes, however can still be seen and shot.

Instead of appearing as bright white silhouettes through a Thermal Scope, Cold-Blooded players will appear as shades of gray similar to the environment, either friends or foes. This may actually cause the player to be at a disadvantage on snow maps, as the gray has more contrast with the environment, whereas the white-hot silhouette of non-Cold-Blooded players is much harder to spot against the white snow (of course, for this reason, most players choose not to use Thermal Scopes on winter maps, so the issue is often resolved). As they are not truly invisible, moving Cold-Blooded players are much easier to spot through thermal scopes than stationary ones. As most Thermal Scope users are more accustomed to bright white blots on the map, it is easy to pass right over a Cold-Blooded user.

The Pro version prevents the red name and cross-hairs from appearing on an enemy's screen when they are looking at the user. It is unlocked by destroying 40 enemy killstreak rewards (of any kind, even UAVs) while using this perk. This is very beneficial to snipers wearing ghillie suits, as they can blend in very well with the environment. If a Silencer is added, this is very helpful to sniper classes, but bear in mind damage is reduced to the point where two shots are required anywhere to the body, unless playing hardcore. The perk is essential for the "ghosting" tactic, and the Pro version can assist in doing so.

Experienced players use the Pro version in order to play dead, often for Ninja Defusing, or just to fool enemies who may have hurt them. By going prone, and repeatedly pressing the weapon swap or "cooking" a stun, flash, smoke or Semtex grenade, it will appear as if the player is dead to others.

Tips

 * Cold-Blooded is extremely useful for anti-air classes, where it is usually used in conjunction with Scavenger and a Stinger.
 * With this perk it is not safe to stand next to someone without it during an enemy killstreak. They will be targeted and the player may be killed with them when they are hit. However, in friendly fire games, the player may be safer, as the average player will not attack players without red boxes since they will assume that the player is friendly. Because of this, players trying to shoot down enemy aircraft with Cold Blooded should stand away from team-mates.
 * Some players prefer to use Cold-Blooded on knifing classes instead of the usual Lightweight, as stealth is an important part of survival.
 * Cold-Blooded is useful for Ninja Defusing as well. Since Cold-Blooded hides the player from any form of detection besides physical contact, or through a heartbeat sensor if the player is not using Ninja, even if the enemy has called in a UAV recon, the player will not show up on radar (though the player will show up on the map when firing their weapon unless a Silencer is used). Without Cold-Blooded, the bomb planter could spot the player on the UAV while he is planting the bomb. If the planter were to catch this, the planter would turn around and either shoot the player to death, or stab the player. In addition to being hidden from radar, the Pro version of Cold-Blooded hides the players name and red cross-hair if the enemy is looking at the player, making it easy to hide in shadows around the map.
 * Destroying air support, disabling sentry guns, wiping out UAVs, and even taking an opponent's care package will count towards Cold-Blooded Pro.

Trivia

 * The perk picture shows a lizard surrounded by snowflakes. This makes sense, as lizards are cold-blooded.
 * There are three ways of obtaining the Pro version of this perk: shoot down enemy air support with a launcher, steal enemy care packages, or destroy enemy sentry guns.
 * To look like a corpse, the player has to have Cold-Blooded Pro equipped and has to constantly switch his weapon or "cook" a stun grenade, a flashbang or a Semtex, but best used is constantly switching to your weapon. This can be used to a objective in Domination, or protect a bomb while being right next to it. Player who have learned to use a throwing knife could use it to silently pick one enemy off. However, keep in mind that if the player does this trick a lot of times to kill enemy player they will try every body which they may think that is alive.
 * Using this perk in conjunction with Ninja makes the player undetectable to all electronic systems (Thermal, Heartbeat Sensors, UAVs, Radar, Killstreak Rewards) but still detectable by players through noise or sight.
 * When killed by a controllable killstreak (Predator Missile, AC-130, Chopper Gunner) the killcam will depict players being killed incorrectly, as it shows a red square around the player's body, even though they would not appear with the square on their monitor normally.
 * Team members using the Cold-Blooded perk will not appear white on a friendly thermal scope.
 * If a player moves his or her cursor over a player that has the Pro version of this perk, and they have aim assist active the player's cursor will slow down when moving over a player regardless of Cold-Blooded Pro, potentially negating the pro effects of this perk to the experienced player.
 * There is a glitch that allows a player to receive Cold-Blooded Pro very quickly. If an enemy helicopter is shot down and spinning out of control, every bullet that a player shoots at the helicopter counts as a killstreak destroyed. It is very effective to use a Light Machine Gun to attempt this, as there is the opportunity to fire potentially hundreds of shots into the helicopter without needing to reload. The XP the player gains through destroying the helicopter has been patched, but it can still be used to gain Cold Blooded Pro quickly.
 * Cold-Blooded Pro along with Ninja Pro with a suppressed weapon will make the user virtually invisible(though visible by sight).This combination can be deadly in the hands of an experienced and cool-headed sniper.
 * Cold-Blooded is very similar to Black Op's Ghost. Ghost only allows the player to be invisible to enemy Spy Plane, or UAV. Ghost Pro is exactly like Cold-Blooded. The player is not only invisible to enemy spy plane, but enemy aircrafts such as helicopters, will not target the player.