Well there are weaponized viruses who could theoretically turn Earth into graveyard (if no countermeasures were in place), but it's more of a deadly epidemic - think bubonic plague but on larger scale. The zombie virus would be pretty ineffective (considering normal shuffling zombies) or impossible to contain (in case of fast/intelligent zombies) which pretty much defeats the purpose of weapon.
The only way I think it could happen is some kind of parasite (there are things like that who prey on ants) which would turn us into husks. Of course that would mean there was some genetic engineering in place, but once again as a weapon it's unpredictable. Think Dead Rising game where the source are wasps
Or rabies. Yup, mutation of rabies a'la '30 days later' when it goes just too fast for a countries to contain. And since they can jump, run fast etc. no normal barricade will stop them. Then again these 'zombies' are much weaker than normal ones, because shots to vital organs work. They aren't dead, just mad. And, more importantly after first impact they start decompose pretty quickly. Considering they lost their intelligence and want to feed only on flesh means at most a month of 'living'. Then of course single bite can start another outbreak.
I do tend to excitedly wonder about all kinds of apocalypses (because dwindling resources and growing population WILL create some kind of crisis if nothing will change), but I'm pretty much sure that I'll be one of the shamblers, or just skull somewhere because I find my survival skills lacking. So while fantasizing is fine the thought of it really happening isn't nice. Not at all.
