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The Infected are normally found walking and are quite weak most of the time, but in huge numbers the Infected are the most deadly foe in the entire game. They are provoked by physical combat, gunfire, use of flashlight, and appearing in their line of sight. When informed of your presence, they will be fast and agile but still can be taken down easily.  The Infected can swarm in massive numbers, even 100 at a time, which encourages you and your buddies (be they AI or human) to stick together. In these situations, the Infected can be difficult to cope with as they usually will come from 2 or 3 different directions. There are a number of different ways to stop the infected, but the best are pipe bombs, Molotovs and fortifying a position and holding the Infected back. A large horde will sometimes be preceded by dramatic music and the distant sound of screaming. These are a special type of Infected. Boomers can vomit on the survivors, attracting swarms of Infected. When Boomers are killed, they explode, forming a cloud of pus and bile, thus also attracting swarms of the Infected.  The best way to deal with them is to shove them away then when they are at a safe distance and kill them. They are surprisingly fast at random moments, and will quickly barge into rooms, vomit on a survivor, and walk out. Keep this from happening by listening out for their belching and gurgling noises, and the occasional heavy footsteps. These are a special type of Infected. A tall Infected that can wrap its tongue around the Survivors and drag them away from their team, the Smokers are typically quite easy to kill but once they have your teammate in their tongue, you need to save them. Fast.  The best way is to kill the Smoker is to melee the player wrapped up, which stops the Smoker dragging the Survivor. Then, take a few pot shots at the Smoker until it explodes into a smoke cloud. The smoke cloud it leaves is a minor obstacle; it distorts your vision slightly and causes your team to cough loudly. You can tell if a Smoker is nearby by their hacking cough and raw-throated scream. These are a special type of Infected. Hunters are a dangerous  type of Infected. They will leap at you at quick speeds and pin you to the ground. When they do pounce on a survivor, the Survivor that is pinned will incur heavy damage which will lead to incapacitation and then death unless a teammate helps them of course.  It's a good idea to melee them off your teammate as quickly as possible, especially if you don't have a good gun in your hands. But make SURE you kill them while they're staggering or they will simply pounce on you again. Hunters can also be stopped by a survivor meleeing them away when they are pouncing. This prevents the Hunter from pinning you and stuns it. It can then be killed easily while it is stunned. You can tell when one is around by the vicious screaming and growling. These are a special type of Infected. Witches are commonly found in areas where Survivors will NEED to go, making them a real pain and hard to avoid. The Witch is incredibly dangerous, she can whack down survivors in one hit and on expert difficulty can kill in one hit.  The Witch is found crying on the floor when found, and if you are clever, you can avoid the Witch and prevent your team fighting her, but there are some situations where you must kill the Witch to proceed (e.g. on a staircase). The witch will always attack the person that provokes her. The best way to deal with her is NOT to deal with her, just let her be and turn your flash lights off. If you HAVE to kill her and are equipped with a shotgun, aim for the head. One well placed shot to the head can kill a witch instantly if all of the pellets hit, thereby avoiding any damage she may deal you or your team. If you do not kill her instantly or are equipped with one of the other weapons and she chases you, keep out of her range and aim for her head. If she reaches her target, she will kill him (Expert) or incapacitate him (any other difficulty). These are a special type of Infected. The Tank is an absolutely massive Infected and is the strongest foe in the game. He is about twice the size of the Survivors, and on Expert difficulty, he is particularly hard to kill. He can pick up blocks of concrete and cars and throw them at you, as well as knocking you away with his giant arms. There is no way to avoid fighting the Tank at all; he will see you and immediately attack your team.  The Tank is quite easy to take down if most or all of your team carry assault rifles. Rapid fire at the Tank using these weapons can take down a Tank in a matter of seconds. Dual pistols are also good weapons to use against a Tank. Another great tactic against a Tank is to throw a Molotov to set him on fire which will drain his health more, but keep in mind that in Campaign or Survival modes, this will greatly increase the Tank's running speed. In Versus you should always try to light the player Tank on fire, as it will actually DECREASE his speed a little. You can tell if one is coming by their thundering footsteps, the ominous music that precedes them or their distinctive roar.

Summary:
Watch our for Infected. Kill Boomers. Avoid Smokers. Look for Hunters. Watch out for Witches. Take out Tanks.