The Boomer –
The Boomer is an obese methane-filled boss infected covered with boils whose main form of attack is projectile vomit, but also explodes when shot.
The Boomer’s vomit is released in one large attack, and in the right circumstances can hit more than one target. The vomit obscures the vision of its victims, and also acts as a pheromone to both existing infected, and causes more to be spawned and attack the victims. This attack has a cooldown.

Huge and obese infected that vomits like a fire hose. Can be shot while strafing. When it hits a survivor with its vomit, the vomit acts as a pheromone to the AI controlled normal infected and causes them to focus their attack on that player. The Boomer gets points for all attacks against the target by infected. Moves slowly. Also, when killed, the Boomer does as his name says and explodes, causing all nearby Survivors to become puked on.

The Hunter –

Since the Hunter is the only boss that can have two spawned in at once, he’s the one you’ll be playing most often, and I think most will be happy about that. His primary attack is a short distance lunge. Charge this attack by crouching, however, and you can fly large distances to land on top of a Survivor and rip him to shreds.

The Hunter is, without a doubt, the most skill-based Infected. Not only do you need to know when and where to strike, but you also need to be able to judge the distance and accuracy of your pounce. Toss in his ability to time jumps from wall to wall, and you’ll find a lot of creative attacks that require timing and practice to get right. The player is even rewarded for long pounces by doing additional damage on impact based on the distance, which is only available in Versus mode.
The Hunter can fill several roles on the team. By jumping around and screaming, the Hunter can cause distractions for the other Infected. Pouncing on a Survivor may buy enough time for one of your teammates to attack. If you can manage long-range pounces, the Hunter is great for dealing out damage to a fast-reacting team. And while all the bosses are great at picking off lone Survivors, the Hunter is the best, as he is the hardest boss to counter on your own. His main weakness, however, is that he is forced to be in the middle of the action. Jumping into a group of four Survivors can be suicide.
The most agile boss infected, able to climb walls and jump far. Attacks by knocking down a survivor and slashing. The only way for the survivor to escape is either by a teammate helping (killing the Hunter) or the Hunter deciding to retreat. Similar to the fast zombies from HL2 Ravenholm rooftops. Wall jumping is possible.

The Smoker –

The Smoker is an average sized boss infected covered in growths and boils and has a fifty foot long tongue as its primary weapon. He also releases clouds of smoke.
The Smoker’s tongue is fired at the head of a Survivor and wraps around their neck. The Smoker can choke its victim from above by hanging them from its tongue. A choking Survivor is unable to move or attack until another Survivor rescues them. During its tongue attack, the Smoker is helpless, even if it misses. This attack has a cooldown.

The Smoker has a 40-50ft tongue that can be used to drag/trip players and even choke them from above. Survivors are able to shoot this tongue off to protect one another. When the Smoker dies, it deploys smoke which severely reduces the visibility of the survivors, thereby hindering their ability to see enemies and survive.

The Tank –
The Tank is a giant beast with an abnormally muscular upper body who relies on his health and strength to pummel the Survivors and lob debris. It is only playable several times throughout a campaign, as this boss is considerably more powerful than the other infected.
The Tank uses its speed and large amount of health to close the distance and slam the Survivors with its massive fists, knocking them back and doing a lot of damage. If the Survivor is knocked down, the Tank can bash the Survivor until they are dead.

The Tank can smash cars to send them flying towards the Survivors. If there is no car around, the Tank will rip a chunk of earth from the ground and toss it.

A huge, muscular infected with amazing strength. Can pick up and throw cars and other large environmental items, smash through concrete walls. Players cannot choose to be the Tank; infected players are randomly chosen to spawn as one. Very loud. When you spawn as the Tank, a frustration meter begins building up for every moment you’re not in the presence of Survivors. If this meter fills all the way up, you lose control of the Tank.

The Witch –
The Witch is a unique boss infected that is only controllable by the AI, the player may not control it.

The Witch will not move until she has become aware of the Survivors. Until this time, she will lay on the ground, moaning. It is possible for the Survivors to sneak by without alerting the Witch, if they move slowly and quietly.
If the Witch is alerted, she will run quickly towards the Survivors. She has a lot of health and is very quick.

These are non-playable AI super-boss infected. They are very rare, only one or two in each campaign. They are the most dangerous enemy in the game. Witches become hostile when you shoot them, shine light at them, or fire weapons around them. They kill with one hit. After killing their prey, they run away.