When you play online versus mode and get your turn at playing the infected, you can be randomly spawned in as anyone of the following infected. Common groupings fro infected are one person as boomer, one person as a smoker and two as hunters. This does vary with upto three hunters at the same time and a boomer or a smoker. The Special infected that you spawn in as is determimned by Left 4 Dead's AI Director.  There is one other Special infectyed that you may or may not get during a level and that is the Tank. When one person gets to spawn as a tank it effectivly gives the infected side and extra Special infected. The playable Special infected are listed first followed by non-playable infected Characters

The fat, slow Boomer vomits a thick bile with two side effects: It blinds Survivors, and it attracts hordes of Infected. When Boomers are killed, they explode, showering bile on anyone nearby. Get close to Survivors when attacking. Even if you miss, they might not.

Hunters are a mutated breed of zombie—faster, fiercer and far more dangerous than common Infected. Keep your ears perked—a shriek from the darkness means a Hunter is near, and ready to pounce in for the kill.

Smokers work best from an out-of-sight vantage point. Ensnaring Survivors from rooftops leaves them helpless and hanging—perfect targets for a pounce or vomit attack from a teammate.

Mutated through infection, the Tank's only strategy is brute force. Find the Survivors. Pummel them to death. Repeat. Healthy Survivors can run slightly faster than you. Try to trap them in tight areas, or use a thrown chunk of concrete to stun them.

Meet the zombie horde: eerily fast, sensitive to loud noises and movement, and able to attack in ravenous waves. They might not notice you right away, but when they do, watch out—they're fast, and it won't take long for them to close the distance.

Witches only want to be left alone so they can enjoy a good cry. They hate flashlights and loud noises, but what they hate more than anything is being shot at. If you hear the Witch's tell tale sobs, turn off your flashlights and try to sneak around her. If she starts to get angry, her growl will let you know she's close to attacking.