There is plenty of evidence to suggest that zombies result from toxicity levels in water and air. Obviously people take precautions but gasmask are never 100% effective.....and the infected, infect!

Zombies aren't always slow and moronic, some (even with half a brain) still possess the ability to use a gun.... maybe not in the way it was intended! Others seem hell-bent on catching you, even when you've run over them a couple of times with your scythed wheels! Most zombies don't even know their fate for hours before turning. When they do eventually give in to the over-powering bloodlust, it plays havoc with their best mates conscience (when he has to blow their brains out!).