The news reports in the background speak of an illness that was spreading rapidly in the population. Burning of dead bodies in the streets etc. It's a sickness that must have infected large parts of the population overwhelming the emergency services and later the military. That it's a bug was all but confirmed by the scientist in the season 1 finale.
Sometimes you don't know. Even the dude at the CDC didn't really know.
The Zombie Survival Guide, which I have on my Kindle) states that is a virus named Solanum.
I still wanna know what he whispered into Rick's ear. I'm sure it wasn't a pecan pie recipe.