While the main obstacle that players will need to overcome in both Left 4 Dead games is the incredibly fast and dangerous zombies, there are also a few slightly more powerful enemies who they will similarly need to deal with, and these are known as the Special Infected. These zombies have mutated in unique ways to grant them a special ability that can prove to be incredibly dangerous if they manage to use it on a survivor, especially if they're separated from the rest of the team.