Through the portal a shipment of crates is carried. Perhaps the people delivering them knew what was inside. Perhaps not. But either way it doesn't matter. All that matter is that
THE CRATES WERE FULL OF HEADCRAB EGGS!!!!!!Now headcrabs are running loose in Marotropolis! They must be stopped before the infestation spreads! Fight the headcrabs and their zombie victims here on September 19th-28th. Over a week of headcrab bashing fun!!!
All headcrabs have a distinct call, so it is easy to tell them apart without even seeing them. All headcrabs have the ability to latch onto the head of a humanoid and turn them into a zombie.
headcrab
The standard headcrab is slow moving and little threat on their own. They can easily be killed by most anything hitting them hard. The only danger is they can leap surprisingly fast over long distances, e.g. onto your head.
fast headcrab
These headcrabs are both faster and stronger than the normal headcrabs. They can cling to walls, jump further and faster than the other crabs, and can run quite fast. They are just as easily killed if you can hit them.
poison headcrab
these walk much slower than the other crabs, but can jump further and will sprint very quickly if injured. When biting their victim they inject a neurotoxin that makes the victim suddenly very weak and tired.
headcrab zombie
The standard headcrab zombie has super-human strength, long claws, and big sharp teeth. They are very slow moving, but can take a lot of punishment before dying. They are capable of living and moving without legs. If you kill the host first, the headcrab will try to attack you. If you kill the headcrab first, than the battle with this zombie is over.
fast headcrab zombie
The hosts of fast headcrabs have all their skin and some of their muscle tissue fall off. They are very fast runners on all fours and can jump great distances and climb nearly vertical faces. These zombies also have longer claws and bigger teeth. They also can crawl around without legs, but are still faster than normal zombies. If either the host or the crab dies, than the other dies too.
poison headcrab zombie
These zombies swell in unnatural ways when infected. They are very slow moving, but serve as carriers for other poison headcrabs. Multiple crabs will cling to the back of the zombie, and the zombie will throw them at any uninfected humanoid in range. They have great strength to make up for their slow movement and are the hardest to kill.
Check out this site to learn more.
http://en.wikipedia.org/wiki/Headcrab