Robert Kirkman, co-creator of The Walking Dead graphic novels, and exec-producer on both TV shows, has already confirmed that he isnt interested in detailing the origins of the zombie apocalypse. While thats probably a wise move, the timeline of the show - taking place at the start of the outbreak - means it will show at the least the spread of the infection, and youd imagine that doctors etc would do some theorising on what is going on, how it started and how it will be stopped. It may not give a definitive answer, but it should give some clues, and provides the basis for plenty more discussion and speculation.