Cold Stream is the third DLC for Left 4 Dead 2.Taking place after Blood Harvest and is considered to be the prologue to The Passing, as both campaigns are connected to each other. The Sacrifice is a three-chapter DLC for both Left 4 Dead and Left 4 Dead 2 and was released on October 5, 2010.The Passing is the first DLC crossover campaign of Left 4 Dead 2, which includes a new campaign and 'new co-operative challenge modes of play' and introduces a new firearm, the M60 the Golf Club, and a new Uncommon Infected called the Fallen Survivor.According to Valve it is meant to bridge the gap between No Mercy and Death Toll. Crash Course is the first downloadable content (DLC) campaign for Left 4 Dead.Valve helped to placate matters, demonstrating that it was still developing content for the first game, including a crossover campaign, 'The Passing', between the characters of both games. The fast turnaround of the sequel was initially criticized by many players of the first game, leading to a temporary effort to boycott the second game.

Valve attributed the short turnaround between the titles as a result of having many ideas to expand on the first game, but more than could be reasonably done through software patching or downloadable content.

Left 4 Dead 2 was released a year later in November 2009.

The game's development was started by Turtle Rock Studios, who were bought by Valve during the game's creation, with continued development occurring at Valve's studios.

Left 4 Dead was released in November 2008.

The Director also makes gameplay dynamic, meaning that no two playthroughs are quite the same. The games are overseen by an ' AI Director', designed to give the players a more dramatic experience based on their performance, penalizing players for stalling while rewarding players with special weapons by taking longer or riskier paths. Certain unique infected creatures pose a more difficult challenge, typically requiring teamwork to take down effectively. Players use a combination of melee weapons, firearms, and thrown objects to fend off attacks from the bulk of the infected creatures, while using an assortment of healing items to keep their group alive. The games encourage cooperative play between up to four players, each taking the role of one of the survivor characters and the computer controlling any unassigned characters. Set in the days after a pandemic outbreak of a viral strain transforming people into zombie-like feral creatures, the games follow the adventures of four survivors attempting to reach safe houses and military rescue while fending off the attacking hordes. L4D 2 features The AI Director 2.0 which expands the Director’s ability to customize level layout, world objects, weather, and lighting to reflect different times of day.Left 4 Dead is a series of cooperative first-person shooter survival horror video games published by Valve.

AI Director 2.0: Advanced technology dubbed The AI Director drove L4D's unique gameplay – customizing enemy population, effects, and music, based upon the players’ performance.

Each of the five new campaigns contains at least one new uncommon common zombies which are exclusive to that campaign.

Five expansive campaigns for co-operative, Versus and Survival game modes.

Over 20 new weapons & items headlined by over 10 melee weapons – axe, chainsaw, frying pan, baseball bat – allow you to get up close with the zombies.

Next generation co-op action gaming from the makers of Half-Life, Portal, Team Fortress and Counter-Strike.

L4D2 promises a satisfying and uniquely challenging experience every time the game is played, custom-fitted to your style of play.

This improved Director has the ability to procedurally change the weather you'll fight through and the pathways you'll take, in addition to tailoring the enemy population, effects, and sounds to match your performance. Helping to take L4D's frantic, action-packed gameplay to the next level is AI Director 2.0. You'll also encounter five new uncommon common infected, including the terrifying Mudmen. You'll be putting these weapons to the test against (or playing as in Versus) three horrific and formidable new Special Infected. In addition to firearms, you'll also get a chance to take out some aggression on infected with a variety of carnage-creating melee weapons, from chainsaws to axes and even the deadly frying pan. You'll play as one of four new survivors armed with a wide and devastating array of classic and upgraded weapons. This co-operative action horror FPS takes you and your friends through the cities, swamps and cemeteries of the Deep South, from Savannah to New Orleans across five expansive campaigns. Set in the zombie apocalypse, Left 4 Dead 2 (L4D2) is the highly anticipated sequel to the award-winning Left 4 Dead, the #1 co-op game of 2008.

Left 4 dead game modes