The modus operandi of a Necromorph is to pop out of a vent and eat your face. With their limbs already being mutilated, they're able to navigate facilities such as the Ishimura and the Sprawl by using the air vents, giving them easy access to any part of the ship and leaving you nowhere to hide. Once you've played enough Dead Space, you'll begin to jump at the sight of a vent, well before anything scary pops out of it, to the point where your friends start thinking you might have a strange form of ancraophobia.