When a human is possessed by a Fast Headcrab, the victim suffers an accelerated degeneration in comparison to victims of standard headcrabs. Much of the victim's flesh and even internal organs are dissolved, believed to be broken down into nutrients for the headcrab. The remaining bone and muscle tissue mutates and becomes harder and the fingers of the host body elongate into long claws which the Fast Zombie will use to swiftly tear prey apart.
Fast Zombies, like other zombie types, generally hunt in packs. Though they are physically quite weak, they are extremely fast and agile. They will sprint on all fours and lunge at their prey, shrieking as they do so.
A single Fast Zombie on its own isn't that much of a threat so long as one is prepared for it, but these monsters rarely travel alone and a pack of them can make short work of a human victim in little time. They are capable of climbing up walls and have a tendency to attack a victim's blind spots, which can often lead to the victim being frightened and stumbling from a high vantage point.
Fast Zombies, although they can be very frightening, aren't particularly dangerous on their own because their attacks are fairly weak. Unfortunately, they often travel in packs of three to four. They're also not particularly hardy - they're just difficult to hit due to their fast and generally unpredictable movement. If facing a Fast Zombie, use the shotgun or Colt Python and aim for the head (the MP7 works well for packs due to the large magazine).
On the rooftops of Ravenholm in Half-Life 2, the biggest threat is actually falling off by surprise when a Fast Zombie suddenly attacks, thus causing fall damage while the zombie pursues. So one should avoid standing at the edge of the rooftop if possible.
Fast zombies are actually much easier to hit at medium range, as it is harder for them to move quickly behind or to the side of the player, and they cannot attack.