If you are playing the tank, be careful to avoid fire traps as persistent burning damage from molotov cocktails tends to be the main way to bleed down his amazing amount of hit points. The Tank is the most powerful of the infected and only pops up in certain areas of the game, plus the player who gets to control him is randomly chosen. Any one special infected by themselves is pretty weak, but if you work in conjunction with the other infected, you can quickly wipe out the four survivors. As it goes, you will only get a few good chances to make this happen, and the key is to coordinate your attacks. Part of the fun in playing as the infected is being able to take out one of the survivors using your special attack. They all have very powerful attacks that are difficult to pull off correctly, but devastating when executed properly.
With the exception of the Tank, the special infected in Left 4 Dead 2 are very weak and die easily. In this Left 4 Dead 2 special infected guide, learn what each type infected can do and how to best use their powers.