Bleeding is a Status Effect, currently only caused by the Scratcher. It takes away 1% of your HP each second when you're standing still, and 3% each second when you're moving. If a zombie gets this status, since it's always moving, it'll lose 3% HP each second. However, when you freeze it (Cryo Grenades, Ronson Cryo Turret, etc.), it'll take only 1% damage per second.