Do you ever wonder why the outbreak seems to spread worldwide so quickly? A bitten person couldn't survive long enough to make it through a plane or boat trip across the ocean before they turned. For example in Diary of the Dead the initial outbreak is somewhere in the United States but 2 or 3 days later the survivors are watching webcasts of outbreaks in Japan. The only way a virus or chemical or whatever caused the zombies to begin with could spread that quickly would be for it to be deliberately dispersed at multiple locations more or less simultaneously (terrorist attack?).