From TF2 Wiki
The Medigun is the secondary weapon for the Medic class. It can heal injured allies as well as disguised Spies, and will boost the total health of the target to 150% (also called overhealing or Buffing). As the Medic keeps his beam focused on a team-mate, his ÜberCharge bar will gradually fill. The ÜberCharge meter will fill faster (2.5%/second) when healing injured team-mates or team-mates whose health is not at 150% (at 150%, 1.25%/second). When the ÜberCharge is used by right clicking after the meter is at 100%, the Medic and his target glow their team's color and become invulnerable for 8 seconds. However, if the Medic stops healing that team member, they will become vulnerable to enemy fire after a short time. It is possible to make two different people invulnerable at the same time by quickly switching targets.
The Medigun beam will stay locked onto its target as long as the wielding Medic keeps the attack button held down and keeps within range of the target. This allows the Medic to turn around and effectively keep an eye out for incoming enemies from behind. In Multiplayer > Advanced, there is an option to have the beam stay locked on without having to hold down the fire button. When a player has been overhealed, a team-colored "aura" follows them, similar to a teleporter trail.
Heal rate is linearly based upon how long ago the target was last damaged. If the target was damaged less than 10 seconds ago, then they are healed at the minimum rate of 24 health per second. From 10 to 15 seconds, this rate increases linearly from 24 up to a maximum of 72 health per second. This mechanic is in place to encourage falling back to heal, as without the boosted healing rate, players often found that fighting, dying, and respawning was quicker than falling back, being healed to full, then re-engaging.
ÜberCharge Fill time is doubled if target is at 142.5% health or above. If more than one Medic is healing the target, each Medic's build time is divided by a factor of the number of Medics healing that target. 
- The Medigun has Developer weapon variants.
- A Medigun Beta 1 exists with the effects:
- -50% heal rate
- Overheal doesn't decay
- The Medigun resembles the nozzle of a fire hose, and functions in a similar manner (extinguishing fires).
- It also has a similar appearance to the famous Proton Packs from the movie Ghostbusters.
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