In your way are the infected, people from all walks of life now insanely enraged at the sight of you. In the hospital they're in backless gowns, at the airport some wear fluorescent jackets. There are camouflaged army infected, policemen, mums and office workers. They're scattered around the levels, depressed, slumped, leaning forehead-first against walls, crying, vomiting.
In isolation they're easy and plentiful targets, popping after a few bullets. One player can easily take down a group of five or six just by spraying them with an automatic rifle. But that's not the point. The infected will come at you in waves. You get no warning of this, unless you accidentally made too much noise or were hit by one of the boss monsters' special attacks. No, there's just a sudden sound cue and these seemingly easy targets swarm. Nothing stops them. If you've climbed on top of a bus, they'll scramble up the sides. They'll burst through doors and walls, and they don't shamble. They sprint, faces contorted with fury, and when they hit they swipe, and grab, and kick. They surround you, a furious mob seeking to drag you down and stamp you to death.
Their vast numbers and angles of attack are what makes the challenge. The surprise element comes from Valve's AI 'director', which does a remarkable job of making these static levels endlessly challenging. In co-op mode it dictates the frequency of swarms, bosses and pick-ups.
It's watching how you play, and planning ahead, either letting up if you need a break or giving you a challenge if you're tooled up and spoiling for a fight. It even watches for panicky moments. If your team is struggling it'll ease off briefly.
If you do get overwhelmed, you have a few options. Spray bullets and hope that you kill enough foes to create room to manoeuvre. Or, right mouse is a combination of melee and push, enabling you to lash out with whatever you're holding, even med-packs (in which case you must shout at the infected "Stop healing yourself! Stop healing yourself!"). Again, the idea is to make room. If you've formed a tight-knit team, now's the time to scream and hope they're able to help. If you're not within real-life shouting distance of the people you're playing with, or if you're not in possession of a microphone, your character yells automatically and your fellow players are alerted by on-screen text. You just have to hope that your friends are paying attention.
To fight off a swarm, your team-mates might throw a pipebomb, a brilliant piece of comedy weaponry that beeps and enrages zombies. The infected will follow it like a cat chasing a piece of string - except with more stomping. There's a lot of visual and aural information in Left 4 Dead. Icons lead you to extra ammo and pick-ups, and warn against doing certain things such as setting off alarms (which will bring a horde of enemies), or angering a boss. Each player also has a glowing outline that activates when they're out of sight and makes them visible through objects, like a wallhack. You're even able to tell when someone's in trouble by the body shape or colour of their outline. And finally, bosses are accompanied by distinctive, unearthly noises to help you ready yourself before they attack. The Boomer's distressing gurgle sounds like a swamp coming to life.
All the while, the game compels you to stick together, constantly asking you to be aware of your fellow survivors' predicament. Find yourself separated from the group and you'll have to take on a boss or a swarm all by yourself. If you're knocked or pinned down you won't be able to escape without another player's help. You'd better hope the others like you enough to save your idiotic ass.
And so we come to the boss zombies: the Boomer, the Smoker, the Hunter, the Tank and the Witch. They're the real threats. The first three spawn regularly, and each has a particular attack. The Boomer tries to ambush you: his vomit attracts zombies from everywhere. The Hunter pounces, pinning you to the ground. And the Smoker snaps out its hideous tongue, tying you up, draining health or enabling other zombies to mob you. For each attack you need the help of your friends. Boomer vomit blocks your view, the Hunter can only be knocked away by another player, and if you miss your chance to kill the Smoker he holds you until you're rescued or killed.