This is according to the timeline of the website...
These people have become infected, and have died before becoming a zombie, the order does not matter. they move slow, and are normally visibly torn apart.
these zombies have not died, but have become infected. Because their bodies have not died, they are still capable of moving quickly, and do not necessarily look disfigured. If a bite wound wouldn't kill faster than the infection would spread, then the bitten will become a Runner.
as for what you're talking about, I've never heard of an "x-class" zombie. Where did you hear it?
hmmm, while I have never heard of an "X-Class" zombie I can posit some hypotheticals:
A zombie that has a higher functioning brain (i.e. can "think")
A zombie that has mutated somehow(Like in video games)
For the purpose of the Site, neither of these exist and should not. I would suggest that the "Campion Virus" shows some level of higher organization by securing the basic function of the lungs (for moaning) and managing to co-opt the central nervous system and musculature to work even without the basic component of oxygen which is necessary in living organisms. It is more than likely that the virus actuallyre-writes DNA on the fly so that the cell structure can be adapted for it's new purpose. Like any living organism, it's sole purpose is to replicate itself. The real question is, will it be able to adapt itself and make the jump to non-human animals? Could this be the X-Class...now that would scare me...
Seriously though, my guess for this "x-class" zombie trip is probably a more advanced recreation of the original zombie. For instance, it probably has the ability to jump higher, or maybe have some kind of mutilated piece of bone or something as a weapon, the ability to run way faster than average, or smarts. Basically what I'm saying, you've got one bad ass zombie that's doubled or even tripled in physical or mental.