The Emperor's Mercy

During the hunt for heretics, Sergeant Tartheros of the Imperial Guard, along with several of his men, are swept over by a mysterious gas. He awakes aboard the medicae on the Alatherum, the ship that had transported him to the world upon which he as inhaled the gas. What he discovers is a startling sight: eight of his men rotting, yet still barely clinging to life. One soldier dies, and as he shows his respects to his fallen subordinate, a mysterious female voice orders him to leave the medicae, somehow overwriting the locks on the door. The man he just watched die somehow gets back up, and begins consuming another sleeping soldier. This is the plague virus, a deadly virus that brings back the dead, only to feast on the living.