I promised to tell a bit about the theory of the zombie bite, and now that I've done a thorough skin check and I'm sure I haven't been bitten, I can discuss this clinically. First of all, the whole zombie thing does not appear to be a virus, or at least not a normal virus. It doesn't affect the living; only the dead. In other words, you can't "Catch" zombie-ism while you're alive, but if you die, regardless of the cause of death, you will become a zombie. This is key because in the beginning, most of the deaths weren't directly caused by zombies. Let me give you a zombie history lesson.
Picture this: Every corpse in a hospital morgue suddenly gets up and starts attacking people. Most of the patients in the hospital are not in any kind of shape to run or fight, nevermind that they have no clue what's going on. The patients get killed and now you've got the morgue zombies and the patient zombies. They quickly overcome whatever staff that doesn't escape, so now you've got the staff zombies. Now suddenly every hospital in the country is not only out of commision, but they've got a population of a couple hundred zombies that start to spread outward. With no hospitals and most of the health care workers dead, death rates go up, creating more zombies. But that's just the tip of the iceberg--now comes the human factor. People start to find out what's going on and they go batshit crazy. Driving like maniacs to try to get to the country or their family or the CDC or whatever. They end up causing accidents, killing themselves and innocents along the way. All zombies now. The criminals and the crazies see this as their chance and they start looting and rioting, killing a few more people along the way. More zombies. The cops finally have to start shooting on sight to get control. More zombies. Your creepy neighbor has always wanted your car, your tv, your wife, whatever--figures this is his chance while the cops are busy. He comes over with a gun and either he shoots you or you shoot him or you shoot each other. More zombies. And once a family member turns, that whole house is gone--who can chop off mom or dad or junior's head?
Now back to the bite. I heard a guy on the news near the end saying that the bite itself was not infectious, but zombies were like kimodo dragons--they carry so much bacteria in their mouths that once you're bitten you will die of a systemic infection in a matter of days (hours?) if you don't get antibiotics. This of course caused a panicked run on antibiotics which resulted a whole new round of killings and a new generation of zombies. So the cause of zombie-ism? Who knows. The cause of the apocalypse? People. Stupid people. Here endeth the lesson.