Left 4 Dead sports a trio of gameplay modes to work through, most with the same premise of getting from saferoom to saferoom while fighting off the "Infected" and "Boss Infected" as a team along the way. The three modes of play are: Campaign, Versus and Survival. Each mode presses the need to work with your fellows in order to succeed and occur in one of the four maps: No Mercy, Death Toll, Blood Harvest or Dead Air.
Campaign Mode[edit]
Campaign Mode follows the story of the four survivors (Zoey, Louis, Bill and Francis) as they shoot, melee, burn and explode their way to some sort of escape from the innumerable Infected in the area that they find themselves in. This mode can be played alone or with up to three other players at one time with strictly Director-controlled Infected and sports four levels of difficulty. It's recommended that players new to the game stop here first and get familiar with the mechanics of the game, as well as how the maps flow before moving onto either survival or versus.
Versus Mode[edit]
In this mode, players are split into two teams of four and fight as either the survivors or the Boss-type infected with the goal being to reach the saferoom at the end of each round with as much health as possible (or at all in some cases where the teams are evenly matched in skill). The infected team has the single goal of stopping the survivors from reaching the end of the stage as soon as possible to limit the number of points scored.
At the completion of each round through the five chapters of a map, points are calculated via the "Health Bonus" the survivors have remaining, times how far percentage wise they pushed through the map (the % is converted into points on a 1-to-1 basis. So if you made it through the map to the safe room with at least 1 survivor, a 100%, then you would be awarded 100 pts), times again how many survivors actually made it without being killed, times the maps difficulty (this multiplier ranges from x1 for the first map, and gradually increases to x2 for the finale).
Once one team plays through as survivors (either by completing the chapter, or by all four players being killed) both teams switch roles and the chapter is played once more in the same manner. After both teams have played through as survivors, the points for the round are compared before the next chapter starts, and the team with the least points starts as the infected first in the next round (this is a general rule, but some modded servers randomize which team is on which side, regardless of points).
Finally, unlike the survivors, infected get multiple lives during versus maps in order to stop the survivors from having just a walk in the park each chapter. Initially, the infected team starts with one Boomer, one Smoker, and two Hunters. Infected don't have to spawn right away and can "ghost" about the map, running at high speeds completely invisible, invulnerable and incapable of interacting with any physical surroundings. When they choose to, infected players must be out of line of sight of all remaining survivors in order to spawn in to deal damage with their respective Bosses. Once one is killed off, an infected player then goes on a spawn timer before they are repopulated back onto the map as a random Boss infected. However, there is a limit to what can spawn at one time. Unless the server is modded, there will never be more than one Boomer or Smoker on the infected team at one time. However, the number of Hunters is not limited to just two, and it's completely possible for the infected team to be made up of all hunters!
Infected also gain the use of the Tank Boss from time to time. One player is randomly selected from the infected team when a tank is about to spawn in, and the team is given a warning saying so. Once spawned in (this is an automatic spawn and is not controlled by the players) the player gains control over the massive Tank's actions, and must engage the survivors quickly due to the Rage Bar mechanic of the tank. If the Tank doesn't attack the players right away, the bar slowly drops on the Tank's HUD. Once empty, the current player loses control of the Tank for a short period, and one of the other three remaining infected players gains control of the same Tank with whatever health it may have remaining. If on the second controlling player's turn the bar empties again, the Tank will despawn. Witches are non-player characters on any versus map, meaning the infected don't get to control her directly, but can use her in a tactical manner (e.g. pulling a survivor via the Smoker into her, thus startling her). Though, there are times when the Witch can be avoided completely by the survivors, much to the dismay of the other team. Witches can still be "crowned" in versus.