Many Zombie Survival Experts warn that the Apocalypse will be started as a result of a large scale bio-terror attack.
The fear is that all it would take is for one geneticist to take something like the bubonic plague and make it as contagious as the common cold, lace a series of bombs with it and place said bombs in select places all around the world.
Imagine if you will, if someone were able to use this type genetic alteration on a creature already in existence in nature to create a zombie parasite. A parasite that removes the fear of death, controls the brain, and exists only to spread to a new host. Now stop imagining. It does not need to be created in a lab, nature already did it. See the video, be warned.