So as you can see zombies are fun to talk about but it's impossible for a zombie outbreak to become reality so there's no need to prepare for a zombie apocalypse. Movies like Halloween and Scream with serial killers sneaking around and killing individuals is far more likely a scenario.
What happens in an I Am Legend setting? Where *spoilers* a cancer treatment is used around the world which mutates them to zombie esque people years down the road. They transmit the disease through the air to 95% of the world population before you know what's going on if you aren't immune you a zombie *endspoilers*.
The only way I could see a real zombie apocalypse succeeding is some crazy virus that lies dormant but still spreads through the air with a really high infection rate when exposed. Think breathing the same air as someone in an office building. A month or two after it spreads through all the major cities in the world the first patients show up in hospitals, and then humanity explodes like an egg versus a shotgun.