The Medic Upgrade is a bonus pickup in Call of Duty: Ghosts in Chaos Mode, which can be purchased from the Armory for use in regular play. When activated, the player will have a sort of aura around them that heals nearby players faster and provides for instant revives.
This upgrade is extremely useful, as it allows the Medic to instantly revive fallen opponents, heavily speeding up the process and making it safer for the Medic to revive a teammate. The Medic needs far less protection to revive a teammate in this fashion, which means that the team doesn't have to spend as much time covering for the Medic. In many instances, using the Medic Upgrade is a plainly better alternative to just simply reviving players as one normally would. The Medic Upgrade is best used when multiple players are downed and need to be revived in short order, although one shouldn't fear using it to quickly revive one person.
The Medic Upgrade is the upgrade that perhaps benefits the most from the Shorter Ability Cooldown upgrade in the armory, as it will allow the Medic to use this upgrade as a superior revival tactic much more often.
Due to the fact that the Medic Upgrade also increases the rate of healing in the group, it is not a bad idea to use the Medic Upgrade when the user or a nearby ally is in severe trouble, especially if a very strong hitting enemy is nearby, although one should try to not use the Medic Upgrade this way unless it is absolutely necessary, as other class upgrades are better equipped for protecting players when a player that is alive is in a pinch. It is also very helpful for allies that are using the Fragile relic, as these allies struggle to recover health that they have lost. The heavily increased healing rate benefits Fragile allies immensely.
Should the Medic be on the verge of being downed, the Medic Upgrade can potentially be used right before going down, and if timed properly, the Medic can revive themselves, going down while the upgrade is active, only to be instantly revived by their own ability. However, this requires extremely specific timing to happen, as most times, the Medic will either use it early enough to not risk seriously going down, or the Medic will use it too late and go down.