On Operation Flashpoint, we only shoot at medics in self-defence. We won't actively target them, and our enemy does the same. Medics do have weapons on our server but they're for self-defense only. We also won't knowingly shoot at anything with a Red Cross emblem on it, but we will shoot at non-incapacitated wounded soldiers who are capable of returning fire. That doesn't mean accidents don't happen, though... We do actively shoot at those helping the wounded, but are not medics.
EDIT: We do wipe out whole squads, though, given the chance. The medic will eventually get killed. At the end of the day, he's still got a gun and is an enemy combatant.
In real life, I feel the same rules apply. There are such things as War Crimes. Don't shoot at medics!
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