Popular mythology states that the only way to truly kill a zombie is to either put a bullet in the brain, or to completely sever the head from the body. Half measures simply won't do: even if you lop off an arm or blow a hole in their chest, there's every chance that they'll keep on coming at you. Valve's latest game has displayed a similar level of resilience this year, and neither a vocal online boycott campaign nor the gargantuan release of Modern Warfare 2 has done much to slow its momentum. And now it's finally here, the last big sequel of 2009.

The zombies themselves - or The Infected, to give them their official name - are probably as good a place as any to start this review. As in the first game, the walking dead are a constant threat to your adventures. There are zombified builders, zombified clowns, zombies dressed in hazmat suits and in a million other fashion styles - and they all want to use your colon as a skipping rope. The level of detail on these rampant rotters is superb, and they look even better when you start blowing them to pieces. The original game was gory enough, but here the Ick Factor is turned up to full volume. Smack a zombie in the head with one of the new melee weapons - perhaps Valve's trademark crowbar - and their skull will burst like an overripe melon. Fire a shotgun into someone's legs and you'll blow clean though their knees. Combat is a messy business: blood sprays, flesh tears, and limbs are left scattered on the reddened ground.

In a sense, this sums up Left 4 Dead 2 as a whole: it's more or less the same co-op FPS game that we played last year, but it's bolder, bloodier, and broader in its approach to the action. The concept for the five core campaigns remains the same. Four survivors attempt to battle their way through a zombie apocalypse, blasting through several interconnected stages in a bid to reach safety. Each player is limited to carrying one large firearm, a choice of pistol or melee weapon, and one single-use thrown weapon. Survivors are also able to carry a limited quantity of healing items - one medkit, and either some short-term-fix painkillers, or a new adrenaline shot that boosts your speed. As before, the distribution of health kits is a vital gameplay dynamic: it's important that you look out for yourself, but if you don't patch up your team-mates from time to time, there's a real risk that the group as a whole will fail.

Valve has added a number of new monsters and features and weapons for this sequel - elements that we'll cover in a moment - but arguably the most important changes lie with the designs of the campaigns themselves. As before, each scenario is presented as a B-movie that places the four survivors in a different environment. Dead Center forces the quartet through a ruined shopping mall, while the excellent Dark Carnival mostly takes place at a theme park. However, this time the levels themselves have greater variety and personality. The Hard Rain campaign abandons the game's traditional, linear structure in favour of a there-and-back-again quest to fetch petrol for your getaway boat, while the aforementioned theme park takes every opportunity to shepherd you into its rides - over a dodgem arena, up a rollercoaster track, and into a shady tunnel of love. At times the first game felt a bit samey, as if you were simply doing the same old zombie killing with a different set of wallpapers, but here each scenario carries its own distinct flavours and set pieces.

This welcome diversity is particularly evident during the climactic events that occur at the end of each campaign. In the past you'd invariably end each adventure with a siege-like stand-off against the undead hordes, a desperate struggle to survive until a rescue vehicle arrived. This time you'll find yourself suffering through all sorts of things: a marathon-length running battle across a suspended bridge; a zombie massacre in a rock stadium; a near-blind sprint over the roofs of flooded houses as you're battered by a tropical storm. The latter is a particularly well-crafted slice of level design, limiting the players' vision and battering them with impressive weather effects - and yet it's not quite the pick of the bunch. The best finale of all is to be found at the close of Dead Center: here the four survivors must race around a multi-tiered shopping complex in search of 16 petrol cans, which must then be brought to a stock car on display in the mall's main hall. The quickest way to gather the cans is to split up the group - but as is usually the case in L4D, this is an extremely dangerous tactic. The resulting scramble is always tense yet extremely enjoyable, and it feels genuinely different to the other parts of the game (with one exception, but we'll cover that later).

The Jockey is almost as freaky as a normal horse-riding midget.

I've spent a lot of time talking about the campaigns and the improved level designs, but it would be wrong for me to ignore the other new features in this sequel. There's now a far wider selection of firearms on offer, and while most of them stick rigidly to the templates drawn up by the original game's machine gun/shotgun/sniper rifle trio, you'll still be grateful for the slight differences on offer. You'll also occasionally get access to special ammo packs that let you fire incendiary or explosive rounds, and for really big bangs there's a rare grenade launcher - a toy that will cause massive arguments if used carelessly. The new melee weapons are far easier to wield effectively, and come in a variety of shapes and sizes. As a rule I find the "slice-y" choices like the chainsaw and katana to be more fun than the "bludgeon-y" picks like the guitar and frying pan, but to be honest they all do the same job - decimating the zombies at close range. Wielding a close-combat weapon prevents you from carrying a pistol, so you'll have to be a little bit more sparing with the ammo on your primary gun.

The final new pick-up, and arguably the oddest of the lot, is a defibrillator unit that can be used to bring recently-deceased colleagues back from the dead. In the games I played, most people seemed to be giving this a wide berth - perhaps because it prevents you from carrying an all-important medkit. Hopefully a few more charitable players will emerge as the online community swells, because in theory this could be a game-saving tactical device.

Naturally enough, the game's cadaverous forces have also gained a few new allies. Aside from the standard Infected, the five Special enemies from the first game have returned, and this time they're joined by three new monstrosities - The Charger, The Jockey and The Spitter . All three fit neatly enough into the game, particularly when you see how their special attacks can work together. Chargers and Jockeys both force survivors into helpless positions: the former dashes into people and then smashes them into the ground, while the latter "rides" people and steers them into nearby hazards. However, it's The Spitter who proves the most dangerous newcomer, with an area-of-effect acid attack that can swiftly decimate an ill-organised group.

A typical situation is that one survivor will be incapacitated, and as a friend rushes over to help them back up, a Spitter shows up and gobs all over the place. In seconds the rescuer's health is whittled down to nothing, resulting in a second incapacitation, while the original victim is killed outright. At this point you'll probably hurl your joypad or mouse and keyboard across the room and make a disparaging remark about your colleagues - most likely a two-word phrase that rhymes with "Mucking Boob".

Such frustration is perhaps one of the only real criticisms I have of Left 4 Dead 2 - and indeed of the series as a whole. There's something unique about the way you're constantly put under pressure by The Director - the neat AI system that remixes level design and enemy placement on the fly - but every once in a while I wish that he'd slow the pace down a bit. The Director's style certainly owes more to Zack Snyder than to George A Romero, and at times the action can seem a little too relentless for its own good. I like a challenge, and I like close-run fights, but I'm not such a big fan of being vomited on by a Boomer just seconds after I've been strangled by a Smoker. This is a notably harder game than its predecessor, and it can be frustrating when you're sent back to a checkpoint for the fourth time in a row.

Still, if you're struggling to get past a certain section, it will often be because of a problem within your group. Left 4 Dead 2 is very much a team game, and to get the most out of it you'll need three other friendly gamers - because playing alone with the AI is as dull as a milk-flavoured milkshake. Unfortunately, "friendly gamers" can often seem as rare as rocking-horse poo, so ideally you'll be playing with people you know. In contrast with most online shooters, where conversation essentially equates to being screamed at by a 14 year old kid from Kentucky. headsets are all but essential here. I know that 93 per cent of communication is non-verbal, but in Left 4 Dead the only thing your body can say is "Eat lead, sucka!" or "Oh bollocks, I'm dying." Actually, that's not quite true: the four characters here are far more verbose than the previous game's cast, and they'll frequently come out with excellent snippets of dialogue. Still, my point stands.

This is Scavenger Mode. It rocks.

Aside from the occasional bouts of being irritatingly aggressive, the only criticism I can make of this sequel is that many of its structural changes are minor. I know there's the whole "If it ain't broke don't fix it," argument, but it's still something to be taken into account - especially if you own the first Left 4 Dead. To be fair to Valve, there are now several alternative game styles to play if you fancy a change from the norm. For a start there's a version of the Versus mode from the first game, wherein one team plays through stages as the Survivors while another quartet plays as Special Infected. There's also a Survival mode where teams attempt to last as long as they can against a never-ending wave of threats. But if you're looking for a hardcore challenge, Realism mode is your best bet. Here you play through a chosen scenario as normal, but without any of the visual aids that help you find pick-ups - or indeed your friends, should you get lost. You foes are also harder to kill, and if a survivor dies then they're gone forever. These changes have a surprisingly large impact on the game, to the extent that teamwork becomes essential at all times. It won't be for everyone, but gamers who like a rough ride will get a real kick out of it.

Finally, but by no means least, we have Scavenger - a competitive team game that turns the fuel-based finale of Dead Center into a four-on-four contest. One team is given a limited amount of time to fill up an engine or vehicle by retrieving gas tanks dotted around the map; the other four players control respawning Specials and attempt to hinder their progress. When time runs out the sides swap over and the game plays out again. The team with the highest score wins the round, and after a set number of rounds an overall winner is declared - matches use a fighting game-style best of 1/3/5 structure.

This may not sound like a particularly big deal, but trust me when I say that Scavenger is one of the best multiplayer experiences of 2009. Obviously a lot of the mechanics from the main game are present and correct, but the pace of the action is faster and more free-flowing. There are fewer standard Infected around and more Specials, and since the latter are now controlled by humans, there's now a need to out-think your opponents. As with Versus Mode, it's incredibly fun to play as the Infected - particularly if you're controlling a Spitter, as his corrosive sputum can set the fuel cans alight. There's a special kind of evil fun that can only be had from ambushing a pair of survivors as they approach the generator, where a well-aimed gob can cause total havoc. Until you've seen a burning man fall over into a pool of acid, you haven't lived.

Aside from being great in its own right, Scavenger shows us that Valve is starting to think about ways to expand the remit of the Left 4 Dead experience. Make no mistake, this is an excellent sequel that does much to build upon the foundations of the first game; however, if there is to be a third entry in the franchise - and I've no doubt that there will be - then Valve will have to go further with its changes. But hey, that's something to deal with in the future. For now, we have Left 4 Dead 2 - a top-quality shooter with oodles of style. And for that, we should be truly grateful.