Hey, soldier, listen up! There's no time to repeat this. The T12 virus has broken containment at the Horizon research station. All the researchers have become walking corpses. The virus is localized for now, but it could break out and cause a disaster at any moment. You're going to walk into that hellhole and cleanse the station. Don't try to save bullets - kill every last zombie.
The station's corridors are full of these creatures. It's a good thing you have a recoilless machine gun. Mop up and come home. Over and out.