My thoughts on the matter are pretty simple, however I do have more then one theory. I will just go with the first one first. I think that the drug companies will start Z-Day with some new drug they are working on and/or testing on animals. Just like with the rage virus starting in the monkey, one will become infected, and it will spiral from there. Before you know it, most people will be infected and the world will be over run by blood thirsty, flesh eating corpses. Whether you are one of the few survivors, or one of the many hungry corpses, you will have your work cut out for you finding and obtaining food. It will essentially be the end of the world, because with mindless zombies eating everyone's brains, and survivors mostly unable to reproduce(hard to survive long enough to gestate) the population will dwindle, and we will become extinct. When all the survivors are gone, what will the zombies eat then? Will they starve? that will be the next question.
Have you ever seen the TV series "Life After People"? It doesn't suggest what causes the humans to disappear but gives a pretty cool accounting of what will happen to things once they are gone.
you know I think I did see that once... I will have to look it up and re watch it though just so I can be sure
I could tell you, but that would ruin the surprise, and probably nullify my potential book deal.
I will continue with my belief... Bored scientists. For some reason bored scientists think its a great deal of fun to inject hamsters with all kinds of things and see how radiation effects them. This time it was ritalin, MSG, and a vein of influenza.