So first we have to assume they are dead, which breaks every natural law as we know them. Then we have to assume in this reawakened state that they have no immune system, which is fair, and no other bodily functions. Then we would have to assume this doesn't cause them to develop bodily toxins, the toxins caused by no longer having a functioning bodily system. Which normally happens for a still moving body with disabled intestines; because, just because. This would take care of all natural predators in for of insects and bacteria. After all that, assuming zombies have any less of an immune system than ours, provided they are dead, which already are beyond magical, that they are eaten away at near impossible speed, so fast that they can't infect others at all.
When we have assumed all of this nonsense, we have to assume that the incests and bacteria would at all react to this new phenomenon, although we know it have taken millions of years for them to adapt to eating what they eat now and that zombies are completely outside the category of everything. Then we have to assume that the virus, especially designed or evolved to do this task has not, at all, evolved to take care of insects and bacteria to survive, which is the case with many viruses. Then we would have to assume insects eat the corpse at superhuman (super-insect) speed, as it takes alot more 20 days ca. for insects to devour a human corpse normally. Far to slow, if it were to prevent the zombies from spreading.
Also, why do you assume zombies, when they are turned, suddenly drops all their cloth and freeze to death and/or lose all body warmth.
Also, it could be very likely that half humanity is already infected, as per the very real Toxoplasma gondii parasite which actually affects approximately 50% of all human, and affect human and animal behaviour. Also that a couple of weeks would be enough. After all, a Canadian scientist's study have shown it would take zombies 28 days to exterminate/spread the virus to 95% of humanity. Although the accuracy of this, too, is very doubtfully. (This later came to inspire the name of an movie and its sequal).
Basically, for any of cracked's theories to be true we'd have to assume zombies breaks all laws of nature, in which case we cannot assume anything. Cracked are for crackhead theories...
Why do we assume biting spreads the disease, when the creator of modern zombies himself (George A. Romeo) said that all humans were already infected. Which would either suggest a airbourne disease or a parasite like Toxoplasma gondii. Also, I have to remind you that the zombie is not the predator, but the disease/parasite/whatever is the predator. And the zombie might mearly be a way to spread the host or speed up the infection process.