When you regain control over the protagonist, start with shooting several infected. Open your inventory and equip the thermal goggles (TDG). They won't help you identify the infected, because you will have to shoot everyone on your way (don't worry about your heroism points, you cannot avoid it).
Descend towards the lowest floors while scouring all of the rooms where the infected may be hiding. After you secure the area, go towards the place marked on your map to exit the quarantine zone and finish the mission.