Move down the path until you find a small entryway into the vents. Proceed inside and search for a ladder in the floor. You'll see Infection Flood popping out of the hole. Spray them with the Rifle, then drop a grenade in the hatch to take down the Carrier below. Then follow the ladder down. You'll be greeted by Sentinel lasers. Drop them, then turn the corner, following the path until it appears to dead end. It isn't a dead end, though. Soon, a door will spring open and Flood will stream out. Fall back and waste them, then make your way through the door they burst open.