What was the cause of the virus that turned dead people into zombies? For example, was it after a nuclear blast or explosion? Was the virus in food or water? Or did the virus surface because of a post apocalyptic occurrence?
This has not been answered in the show.
In Series 1 Episode 6, Rick and his group
reach a Center for Disease Control facility
where they meet Dr Edwin Jenner, a scientist who has been trying to understand the zombie virus. Jenner has more scientific knowledge than any other character in the series so far, and has closely studied the virus; but he doesn't mention anything about its origins, probably because he doesn't know.
However, we can rule out a "nuclear blast or explosion" as the cause of the plague. A large-scale nuclear detonation is impossible to conceal and would be international news, but none of the characters has ever mentioned such an event.