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.
-
[spoiler]it is a virus. everyone is already infected and a carrier. it takes death to make someone turn. bites cause sepsis and massive bacterial infection due to the fact zombies are decaying flesh with all kinds of decomposing bacteria [think komodo dragon/monitor lizard spit], and even a normal mouth has many kinds of bacteria.. [/spoiler] well if you haven't read the comics, i suggest getting the compendiums and novellas...spoilered for those who haven't.