There is no official answer atm but what do you think anyway. How did it start, what caused it? Trivia (of the linked page). - "In the Webisodes, it is rumored that terrorists caused the "infection." This is most likely not true, as Kirkman himself never intended to explain the source of the outbreak." Do you think they will ever reveal how the zombie pathogen was caused and how it spread? You think the Zombies will all die off? Oh and check the FAQ in the link for valuable answers for questions which are frequently posted. They are answered by the authors/directors themselves.