Robert Kirkman has repeatedly stated that he will never reveal how the virus originated, so all we'll be getting in this spinoff is the chaos that happens during the outbreak. Which is fine by me, because it heightens the fear and uncertainly in the show.
From the original series, the CDC doctor at the end of episode 6 in Season 1 whispered to Rick that "It's in your blood. We're all carriers" and Kirkman has written this in the comic books, which he states applies to the TV series as well:
"The rule is WHATEVER it is that causes the zombies, is something everyone already has. If you stub your toe, get an infection and die, you turn into a zombie, UNLESS your brain is damaged. If someone shoots you in the head and you die, you're dead. A zombie bite kills you because of infection, or blood loss, not because of the zombie 'virus.'"
So death triggers the reanimation into zombies; a bite just makes the process faster.