[TennTuxedo]

"An interesting twist could be that the zombie virus is spread through intravenous drug use like other things spread via bodily fluids. It spreads nationwide via the drug trade and drug smuggling, which is why it pops up all over the place AND goes uncontrolled for so long. The violence of people attacking and eating each other won’t stand in a suburb, but in a drug den or homeless shelter, people think it is “bath salts” or another bad high and let it go, ignoring people who rise from the dead until it is too late to stop another few getting infected. Until the strain goes airborne or somehow otherwise gets into the whole population."

Remember Jenner from TWD at the CDC? He told Rick that everyone is a carrier of the virus and that when everyone dies, they will too rise from the dead.

[tbw2]

I remember the scene, and that explanation tells us why everyone who dies rises.
It doesn’t tell us how such a pathogen spreads throughout the world (or North America, as far as the show tells us) so quickly.
After all, a virus in one region that causes the dead to come back would get noticed and suppressed unless it was nearly simultaneously everywhere.
Otherwise, you nuke the infected area or fire bomb it Dresden style to wipe it and the zombies out.

[Mad Dawgg]

True but it still could have started with virus laced drugs. It would be an excellent vector. O.D.s are rushed to the hospital and then treated with stomach pumps etc. If you had a highly contagious airborne virus getting it into a hospital under the guise of an O.D. would defeat most precautions against an airborne communicable disease.

[Brooklyn Attitude]

“Remember Jenner from TWD at the CDC? He told Rick that everyone is a carrier of the virus and that when everyone dies, they will too rise from the dead.”

That was late in the outbreak. Remember the scene when Michonne and Andrea first meet the Governor and they didnt understand why he stabbed the chopper pilot. In their experience the only people that turned were bitten.