The medic plugin is a serverside mod that allows players to heal themselves when their screen turns red. Type !medic in the chatbox to use the plugin, or open the console and type Bind "key*of*choice" "Say_team !medic" This automatically makes you "type" !medic into the chatbox, and also sends it.
The !medic command heals you with 50hp, a normal rifleshot does about 80 dmg (will therefore bring you back to 70hp), unless its a headshot. This means that !medic makes the "hurt-screen" (activated at 30hp) disappear, so that you can continue fighting. The code for the WW1 Medic plugin is mostly lifted from an old Day Of Defeat: Source plugin, which becomes apparent after decompiling the source code. (Snippets of the old code are still left unaltered)
Please note that this is a 3rd party script, and is not used in all servers.