To keep the game constantly fresh, and to add to its replayability factor, all spawning of enemies is handled by a dynamic "AI director," which acts like a virtual game master. Though the physical layout of the game's scenarios remains constant with each playthrough, enemies will appear in completely different locations at completely different frequencies. During our first run-through, a massive zombie horde was waiting outside the starting building; the next time, we found only a few scattered enemies. The game is constantly evaluating player performance and pace in order to provide an appropriate level of zombie resistance. If your team is struggling, and constantly taking a good deal of damage, the AI director will scale back on the zombie spawns. If your team is chewing through zombies without breaking a sweat, the AI director is much more likely to spawn large hordes and toss in a few bosses.

While Left 4 Dead's standard infected hordes are fairly single-minded, straightforward enemies, its five variety of boss infected exhibit much more variety. The Smoker, reminiscent of the trap-like Barnacles from Valve's own Half-Life games, is able to grab and drag players with his 50-foot tongue. Unlike his Half-Life counterpart, the Smoker can pull his victims in any direction--even down, given certain circumstances--which can be quite startling. The catlike Hunter is able to cloak himself, and can leap incredibly great distances with great speed and agility. The rotund, corpulent Boomer has a surprisingly long-ranged projectile bloody vomit attack which obscures his victim's screen and makes the player the primary target of all nearby zombies. The hulking Tank is simply extremely powerful; his melee attack does massive damage, and he is able to hurl cars and destroy walls, much like a zombie Kool-Aid Man might. Finally, the Witch tends to sit silently unless disturbed, at which point she unleashes hell.