Versus

Versus is the basic competitive mode for the Left 4 Dead series. 2 separate teams of 4 players take turns playing as the Survivors and the Infected competing for the most points. In Left 4 Dead, points are only awarded to teams playing as the Survivors, however, in Left 4 Dead 2 teams playing as the Infected get points for incapacitating 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 game consist of 2 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 to the safe room at the end of each chapter with at least 1 of the 4 Survivors. The objective of the Infected team is to incapacitate (or kill) as many Survivors as possible. Up to four human players can play on each team, however, while the Survivors can have AI players, the Infected team will only have as many Special Infected as there are players. AI 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 AI controlled) or if the Tank becomes too frustrated (it will also revert to being AI Controlled). Some maps are also slightly changed to make it harder for the Survivors, such as removing closets or areas where similar tactics are possible.The teams switch sides as soon as the Survivors reach the end or they are killed. At the end of each turn the Survivors see the score. 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 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. Survivors do not respawn in closets when they die, unlike the campaign mode.

Infected
The player controlled Infected respawn after a certain amount of time which depends on the amount of Infected players there are. When there are four Infected, players can wait up to 30 seconds to respawn, and a team of one single Infected can respawn in around 5 seconds. After waiting out the respawn time, the Infected player will enter spawn mode which is indicated by a blue aura on screen. In this time, the player must choose a location where they want to spawn, and this must be out of sight of Survivor players, and a certain distance away from them. The Infected player can only run and jump around the map in spawn mode, and they cannot be seen by the Survivor team. Other Infected see spawning players as a blue outline. Once the Infected player has spawned they are active and play as their Infected role to attack the Survivors. When the Director chooses what Infected the player becomes, it EXTREMELY rarely picks 4 "pinning" Infected. "Pinning" Infected are the Smoker, Charger, Hunter and the Jockey. 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, they play a support role; the Tank is a special spawn, it happens usually once per level and control is given to the player on the Infected team with the most damage already done to the Survivors. You cannot be the Witch or common infected.

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 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 4 medkits 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 pills and temporary health contribute.

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


 * $$( P + H ) × N × S $$


 * $$P$$ = 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 his/her 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 him/her, 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 following formula:
 * $$MyHealthBonus = floor( Permanent Health / 2 ) + floor( Temporary Health / 4 )$$
 * 5. Add all four Survivor's health bonus, this became 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 and the Infected side can also score points during the round.
 * The distance traveled by each Survivor is the primary source of points for the Survivors.
 * Every Survivor that makes it to the safe house adds 25 points to the score.
 * The health 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.
 * In the event of a tie at the end of a round (eg, all Survivors on both sides make it 100% of the way), a 25 point tiebreak bonus is given to the team who did the most damage as Infected
 * When the game was first released, the Infected got 10 points each time they incapacitated a Survivor. This has been removed with a recent patch.

Trivia

 * Originally, the only mode was to be Versus. However, Valve found that it was very difficult to balance the game so that both teams could have fun, so they split the game modes.
 * In addition, in versus, the player had the ability to select which Infected he wanted to play as. This menu can be seen in early footage.