Watching High School of the Dead and having played the Left4Dead games for over 500 hours combined, I sometimes wonder about the types of zombies / infected represented in those two titles and in other media as well. This post are just my thoughts on stuff like realism (lol), speed at which the infection spreads and other little details.
The Zombies portrayed in HOTD seem to be your classic run-of-the-mill zombies: If you’ve been bitten you die a few minutes later and come back to life as a slow moving, flesh eating zombie. Details like them being blind and not being able to feel pain are optional and have been added to the standard recipe, just like the “if it moves like us, don’t attack it” in the street-crossing scene from Shaun of the dead. 
At least it was like that for the first five episodes. In episode 6 we see a mother being bitten by her child she’s being carrying over the bridge. She turns within seconds, probably without loosing consciousness, because we don’t see her falling to the ground or standing up after she turned. It’s bite, cut to policemen, cut back to her, already being a zombie. To me, it feels like the characters of the show suddenly are dealing with a different kind of infection, maybe the infection, whatever it is, mutated to be able to turn a person without killing it first, maybe it has been able to do this from the beginning, depending on the physical condition of the victim, killing those with a weak immune system before turning them, while the bodies of people with are strong immune system are being able to put up a fight, until the brain has been taken over (or something like that), but it probably was just the staff not really thinking about it / adding it so idiots on the Internet can spend hours thinking of theories how this is possible… oh… 😦
So, there are those classic zombies and there are the more modern ones from movies like 28 days later and Zombieland and games like left4dead. The biggest difference between these zombies and the old ones is that the new zombies are able to run. They still are able to use a large part of their motor abilities and sometimes even seem to have something what can be called a consciousness. This kind of infection doesn’t necessarily kill it’s victims to make them join the team Z, most of the time, it just puts them in a state of constant rage or affects the human consciousness in a way that overrides reasoned thought and makes instinct more powerful when it comes to controlling the body of the victim, not like the classic infection with the virus suspending the laws of physics and biology. They still move like human beings, but with a animal-like influence.
Now to the speed of the spread of the infection: One thing I couldn’t get my head around in the first episode of HOTD was that those zombies have overrun the entire school within what has been portrayed as mere minutes, the time it probably would have taken them to move from the school-gate to the main entrance. It didn’t make much sense to me.
Now if a zombie-infection with those slow-ass zombies were to strike and the occurrence would have been reported quickly and the officials act accordingly a wide spread, country-wide infection like portrayed in many movies and in HOTD seems highly unlikely. Moving in the troops, mowing down the X000 infected, done. Matter of a few hours. This, of course, only applies for an infection that starts at a certain ground zero. If the first infections were to strike in several locations at once, things could become far more complicated, depending on their numbers and the forces to fight them.
For the new zombies things would be more tricky, with them being able to run and to climb, the humans find themselves in a situation in which it’s much trickier to survive. Probably only few people would be able to outrun a group of zombies chasing them and with the zombies’ higher speed, the infection is also able to spread much faster than the classic ones. To be able to nip the infection in the bud, the reaction by the armed forces needs to be really quick and efficient, otherwise civilization will go down the drain.
Not only they pose a bigger threat to the humans, the new, fast zombies seem much more… uhh… realistic than the old ones. To me it seems like a disease causing raging insanity and that somehow makes more sense to me than the bitten dying and then only being able to kill them by destroying the brain.
So: fast zombies are scarier than slow zombies.
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