Versus mode is much like the base game of Left 4 Dead, only with other humans controlling the opposite team instead of the Infected being controlled by the AI. There's still the director, who controls what specials spawn, and both Witches and common Infected can't be played by the players. It's also a game of score. Each campaign done in VS mode follows the following steps:

• Campaign starts with the Human and Infected teams from the lobby playing the particular side they chose.
• Humans must try and make it to the end of the level without dying once, and Infected must attempt to eliminate the Humans as quickly as possible. Yes, without dying at all. There are no respawns in Versus, sorry.
  
• Upon finishing a level, the Human and Infected teams switch places to repeat the process.
• Once both teams have finished the level as Humans, the next level is loaded and the whole cycle repeats.
• Upon completion of a campaign, the score is tallied for each team. Whichever team got the highest score wins the campaign.

Scoring is based on several different criteria, most of which are from the Human side. You can still get points for your team as an Infected, though, so don't give up! You also deny the other team their points if you kill them, and just attacking them reduces their points at the end. Awesome!

• How far the players made it through the level before dying or finishing. This is the average distance traversed by all players, meaning you can finish a level with less than perfect in this category. Gives 1-100 points, depending on percentage completed.
• The sum of the healths of the Human team. If they are all at perfect health, it's 400 points in the bag. If one makes it in with 1 HP, the Human team gets 1 point.
• How many survivors made it alive to the safehouse. The number of survivors is the multiplier that is applied to the score. Goes from 1x to 4x.
• The difficulty of the map. This ramps up as the campaign continues, and, like survivors making it to the safehouse, is expressed by a multipler. Goes from 1x to 2x, including decimals.
• The damage done to the enemy team as an Infected. This doesn't translate perfectly, but doing well as an infected will indeed boost your score some.

While you may see score multipliers in the game itself, they can't actually affect the score itself. What they appear to do is increase the amount of damage of whichever player got it. For example, barfing on players with the Boomer will cause your multiplier to skyrocket, as you get assist points for Infected that attack barfed survivors. In reverse, defending your buddy from an Infected about to tear his face off will increase your damage as well.

The basics of Human play are pretty easy, as it boils down to "play the game like you do normally." However, the Infected team is much harder to play, thanks to the higher health and firepower of the survivors. It can be a little frustrating for those new to the gametype, so here's a handy-dandy look at the basics of playing Infected as well as how to play any of the four possible classes.

Always stick together
Birds of a feather flock together. Infected that bleed pus gather on the decrepit bus. Whatever. This is the most essential rule of playing the Infected. Never wander off alone or try and go at it Rambo. The Survivors outnumber you, and each one is easily a match for you if you don't managed to snag them immediately. If you are in groups, it turns less of an Infected hunt and more of a zombie movie for the poor survivors. Infected that run off doom their team to failure.

Use strengths to your teammate's advantage
This one bears mentioning because not many people live by it. You do much better in a combined arms assult on the survivors than just using your benefits to yourself. For example, as a Smoker, drag a player into a Boomer so he can barf on them. That's good teamwork that used your two strengths together to take out a survivor. There's plenty of different combinations for this to work, including ones that use the whole Infected team to take out the whole Human team. Learn them and practice them. They are your bread and butter.