Headcrab Zombie

The Headcrab Zombie is the most common zombie you will find. It was added at the very beginning of the game.

Description
Headcrab Zombies are alive humans used as hosts for the parasites known as Headcrabs. Recently taken zombies will be on the ground for some time before getting up. During this sequence, zombies are immune to all damage. Headcrab Zombies are also the only known zombies that can mutate. The first mutation is the Gonome, which takes some time before mutating, then Mr. Friendly, which requires Gonomes to consume dead bodies before transforming into it.

Headcrab Zombies are slow, and easy to kill. Careful, zealous placements of a bat to a Headcrab Zombie's head can easily defeat it. Upgrading this creature will make it more formidable, and therefore take humans looking for easy kills by surprise.

Strategies (As Headcrab)
To become a Zombie, you must first jump on a human's head. After 30 seconds you will get up and be a zombie, however often people will camp until you get up and they can kill you. Remember that zombies are very slow and it is hard for a zombie to escape dangerous fights. The best method is to attack individuals or smaller groups (2-3 people) in groups (or alone). After killing a human, you can eat it by holding down E, which regenerates HP.

Strategies (As Human)
Fighting Zombie Headcrabs is not difficult. Even a regular gun can defeat this creature, close combat is possible but very risky. It is best to fight up close when the Zombie is low on HP. You'll find it easier to distance yourselves from zombies and then start attacking them with ranged weapons.