Versus

Versus is the basic competitive mode for the Left 4 Dead series. Two separate teams of 4 players take turns playing as the Survivors and the Infected, competing for the most points. Points are only awarded to teams playing as the Survivors. Before a game starts, the lobby leader chooses a specific campaign to play and the players can choose which team to play on by choosing whether to start as Infected or Survivors.

Versus Gameplay


Each Round of a versus game consists of two turns. One team plays as the Survivors and the other plays as the Infected. When the turn ends, the teams switch sides. As Survivors, the goal is to make it as far as possible in the chapter before being overwhelmed by the Infected (ideally, all four Survivors will make it into the chapter's safe room). On the other hand, the objective of the Infected is to incapacitate or kill all of the Survivors as fast as possible in the chapter.

Up to four human players can play on each team; however, while the Survivors can have A.I.-controlled players, the Infected team will only have as many Special Infected as there are human players. A.I. only occurs in specific situations, generally being when a human player spawns as the Tank (the Special Infected that they were playing will revert to being A.I. controlled) or if the Tank becomes too frustrated from not being able to see the Survivors in time; control of the Tank will be given to someone else on the same team. However, an update in Left 4 Dead 2 has added Special Infected bots in its Versus mode, so now it is possible to have very few players (two players minimum, one on each team), while bots on both teams fill in the missing player slots; this way Versus games can be fair and even. Compared to Campaign mode, the Infected also attack and deal damage faster. Both teams need at least one human player in order for the game to start or continue properly; otherwise, the game ends and all remaining players are returned to the lobby (even if a player switches teams before the server completely shuts down or unless sb_all_bot_game is set to 1). Some maps are also slightly changed to make it harder for the Survivors, such as removing closets or areas where Survivors can hold out for an indefinite amount of time.

Since each chapter of a campaign is played through twice (once for both teams), it should be noted that all the objects and some Special Infected (the Tank and the Witch) that spawn in the chapter will be nearly identical for both sides (i.e., weapons, healing items, and other objects will spawn in exactly the same place for both teams when they play as Survivors, and the Witch and the Tank will usually spawn in similar places for both teams, though slight variation does occur). The teams switch sides as soon as the Survivors reach the end or they are all incapacitated or killed. At the end of each round, all players see their current team scores. Whichever team currently has the highest score will play as Survivors first in the next round. In the event of a tie, the team that played the Survivors first for the previous round will play them first again in the next. Once the campaign has been played through entirely, the final scores for each team are displayed, and the game returns to the lobby. For help as Infected or Survivors, review some tactics.

A hidden statistics screen was discovered using the console on the PC version of Left 4 Dead. It shows how much damage you did to specific Survivors, and information specific to the Infected you were before you died. It is not implemented in Versus mode.

Survivors
Survivors will spawn at the beginning of each chapter as usual, but they will not retain any weapons, health, gnomes, or items from the previous map, and they will begin with Tier 1 weapons at the beginning couple of chapters, and usually Tier 2 weapons during the last. Items from the safe room that were used during the previous round are respawned as well. Survivors do not respawn in Rescue Closets when they die, unlike the Campaign mode. However, in Left 4 Dead 2, a dead Survivor can be brought back to life if another Survivor happens to have a Defibrillator; this results in a Defibrillator penalty on the Survivor team's score.

Infected
Player-controlled Infected can spawn in any unblocked space, provided that they are not too close to the Survivors or too far from them, they are not in the Survivors' line of sight, and they are not in a restricted area (such as the safe room at the end of a map, or the starting safe room prior to the Survivors vacating it). After they are killed, the player-controlled Infected respawn after a certain amount of time, which depends on the number of Infected players there are and the length of time since the player's last spawn. The respawn timer appears a few seconds after the player Infected is killed. When there are four Infected, players can wait up to 30 seconds to respawn, although the average is closer to 20 seconds. Originally, a "team" of one single Infected player could respawn in around 5 seconds, although later patches added AI-controlled Special Infected to fill empty slots on the Infected team. The spawn timer remains at only 5 seconds for single players on the infected team, however.

After waiting out the respawn time, the Infected player will enter spawn mode which is indicated by a blue tint on screen. The player must choose where they want to spawn. The Infected player cannot be seen by the Survivor team. Other Infected see spawning players as a blue outline. Unspawned "ghost" infected can move through destructible objects and physics objects, such as doors and certain barricades. These unspawned Infected cannot be harmed by other players, either Infected or Survivor, although they will be killed without spawning if they fall into instant-kill zones (such as off a bridge into water); Infected who "die" in ghost mode restart their spawn timer as though they had actually died, and will respawn similarly. Once the Infected player has spawned, they are active and should attack the Survivors.

The AI Director determines which Special Infected "class" each player will spawn as once their spawn timer runs out and they enter spawn mode. The Director rarely chooses a combination of four "pinning" Special Infected. "Pinning" Infected are the Smoker, Charger, Hunter and the Jockey (or any other combination, such as three Hunters and one Smoker). This is so the Infected team cannot simply pin all four players at once, ending the round. The Spitter and the Boomer do not pin in their attacks; instead, they play a support role to the other Infected (Special or otherwise). In Left 4 Dead, the Director will typically give the Infected team two Hunters, a Smoker and a Boomer. In Left 4 Dead 2, the Infected team will almost always consist of four different Special Infected. While this is the most common configuration of infected, there are occasional exceptions. The Tank is a special spawn, randomly occurring, with a maximum of one instance per level aside from mandatory Tank spawns in finales and other fixed locations. Control of the Tank is given to a player on the Infected team randomly, with the odds of selection increasing with the player's score. You cannot play as the Witch, Common Infected, or Uncommon Infected.

Some of the Special Infected also have different attacking mechanics from Campaign mode in Normal:


 * The Boomer can aim upward to let the puke fly in an arc, splashing the Survivors from a further place, sometimes even from where the Survivors cannot see him. He can also move his aiming reticule mid-puke to alter the trajectory of the outbound puke; doing so can let him hit Survivors who he had initially missed.
 * The Hunter can now deal damage on impact by pouncing: the further they jump into the air, the more damage they do when they hit (up to a 25-point maximum). The pounce will also stun the nearby Survivors. Experienced players can also do multiple wall jumps to attack the Survivor from higher positions; however, they lose the ability to jump away instantly when being attacked.
 * The Smoker now deals damage when he is dragging the Survivor.
 * The Spitter's acid can now destroy scavenge-style gas cans (including the finales of Dead Center and The Passing) that have been picked up and dropped by a Survivor. The auras of these gas cans will show up as white before they have been picked up, and as yellow once a Survivor has dropped them.
 * The Jockey's leap can also stun the Survivors just like the Hunter's pounce.
 * The Tank has 6000 health points. The Director will also spawn them on the map when the Survivors enter a designed area, and let an Infected player take control immediately (in Campaign and Single Player mode, the Director will put the Tank on the map and let the Survivors alert it by themselves if not in a finale). Originally, the Tank was slowed down while on fire (in Left 4 Dead Campaign and Single Player modes, the Tank will actually move faster when on fire); this loss of speed from fire in Versus was removed by patches in both games in October 2010.
 * When the Tank spawns, control of the Infected the player controls is overridden. The Special Infected will then turn into an AI Infected if already spawned.

In Campaign and Single-Player modes below Advanced difficulty, the Special Infected cannot attack immediately upon seeing the Survivors; in Versus Mode the Specials do not suffer this restriction unless they are AI controlled. In Versus, the Infected also have the ability to spawn any time after the Survivors leave the safe room (in Campaign and Single Player, the Director will generally stop spawning Special Infected when the Survivors enter the crescendo event area or the finale area, but have not yet started the crescendo event or the finale. This is for Survivors to set up and prepare). Player-controlled Infected can even spawn in the starting safe room once all Survivors have left it, but unlike AI Special Infected, they cannot spawn inside the safe room at the end of a map.

Controls of the Infected
Each Special Infected has their own unique special attacks, but most have similar attack controls. The Special attack is used with the left-click button on the mouse or Right Trigger on the Xbox 360, and the Melee attack is the right-click or Left Trigger on the Xbox 360. The Tank's attacks are in the reverse order, as his melee attack is intended to be his primary means of attack.

The Infected player can also see Survivors or other Infected players by their outline:

The outline will appear when Survivors have recently performed these actions: These action will not give away Survivors' location:
 * Survivors who have health over 40 (permanent and temporary) will have a green outline.
 * Survivors who have health between 10 and 39 (permanent and temporary) will have a yellow outline (between 25 and 39 health in Left 4 Dead 2).
 * Survivors who have health lower than 10 (permanent and temporary) or incapacitated will have a red outline.
 * Survivors in Left 4 Dead 2 who have health lower than 25 will have an orange outline.
 * Survivors in Left 4 Dead 2 who have exactly 1 health after being incapacitated without healing or are currently incapacitated will have a red outline.
 * Survivors who are covered in boomer bile, regardless of their health, will have a purple outline.
 * Infected Teammates, spawned or not, will have a blue outline.
 * Docile Witches will have a white outline.
 * Witches in the process of being startled will have an outline that slowly changes from white to red, depending on how close the Witch is to being startled.
 * Startled Witches will have a red outline.
 * Shooting with any weapon.
 * Using their melee weapon.
 * Using their Melee shove.
 * Using consumable items.
 * Moving without crouching or holding the walk key.
 * Being pinned, constricted, pummeled, ridden, or covered in boomer bile.
 * Under attack.
 * Using a microphone.
 * Talking - either by scenario, interaction with the environment or Infected, or voice command. Note that when Survivors are low on health, they will constantly comment about their health condition and give away their location.
 * Using the chatter box. Note that the outline will appear once the box is active and will last until the message is sent.
 * Reloading; however, Survivors will sometimes yell out "Reloading" which will give away their location.
 * Jumping. Note that moving horizontally mid air without crouching or holding the walk key will still give away their location.
 * Moving while crouching or holding the walk key.
 * Opening or closing doors.
 * Picking up, releasing, or throwing items. Note that Survivors may yell out the item they are picking up or throwing and give away their locations.
 * Starting a Crescendo event or Finale. Although some Crescendo events require player to shoot, melee, or talk, and will still give away their locations.
 * Staying in sight of a Special Infected; the Infected team does not share their vision of Survivors.

Left 4 Dead
The scoring formula for Left 4 Dead ' s Versus mode has not been made public by Valve, and is often under dispute by players. Arguments often start when players heal themselves or weaker players, or swap Pain pills before shutting the safe room door, especially if another player shuts the door before they're done. It's been generally agreed that healing one's self before shutting the door is unnecessary (or may even decrease the score), since teams with low health and four First-aid kits will usually score above 160. It's not yet been agreed whether healing a weaker player will help the score and how much if at all Pain pills and temporary health contribute.

A rough formula for a team's score each round is:


 * $$( P + H ) \times N \times S$$


 * $$P$$ = Average distance the Survivors traveled, between 0-100%


 * $$H$$ = Health Bonus. The Health bonus is calculated with the following rule:


 * 1. Each Survivor scores at most 50 points of health bonus.
 * 2. If the Survivor has pain pills in their inventory, they will be automatically applied (prior to health kit) to the owner as temporary health.
 * 3. If the Survivor has a health kit with them, it will also be automatically applied to the owner. This also overrides temporary health the Survivor has.
 * 4. The health bonus for each Survivor is calculated with the following formula:
 * $$MyHealthBonus = floor( Permanent Health / 2 ) + floor( Temporary Health / 4 )$$
 * 5. Add all four Survivors' health bonus, this becomes the health bonus recorded.


 * $$N$$ = Number of players that survived. If the number is zero, the game treats it like a 1 and the score is unchanged.
 * $$S$$ = That level's score multiplier, which increases as the game progresses.

Note that if the Survivors don't make it to the safe room, then their score will be P × S. This fact has led many players to "go for distance" when they feel that all hope is lost in a round.

Left 4 Dead 2
The scoring formula in Left 4 Dead 2 is different from that in Left 4 Dead.


 * The distance traveled by each Survivor is the primary source of points for the Survivors. The farthest distance a Survivor reaches is the score that Survivor gets (i.e., if a Survivor backtracks and dies, they will receive points for how far they traveled before backtracking).
 * Survival bonus points: Every Survivor that lives at the end of the chapter gets a 25-point bonus for their team. If the Survivor makes it into the safe room and dies, the 25-point bonus will not count.
 * If they are in the safe room and incapacitated, the Survivors will still receive the bonus. However, if the incapacitated survivor is in the rescue vehicle at the end of a campaign and all standing Survivors are in the vehicle, the vehicle will leave and the bonus will not count.
 * The health or items of the Survivors no longer affects the score.
 * The Infected and Survivor sides are notified at 25% intervals of the distance the Survivors have traveled.
 * Reviving dead Survivors using a Defibrillator results in a 25-point penalty.
 * In the event that both teams reach the safe room with no deaths, a 25-point tiebreak bonus is given to the team who did the most damage as Infected. If both teams earn the same score in a level, but not everyone makes it to the safe room (i.e. someone dies), this tiebreaker does not occur.