Glad you're enjoying the show. TWD comics & tv series decided not to focus why and how the dead became walking. Nor the early stages of the virus. and aftermath.
TV producers have said see the 2004 version of Dawn of the Dead for that.
BIB - I would LOVE to see the TWD team take a look at that, perhaps as a flashback episode/webisode or something. The Walkers are terrifying enough to those now used to them - imagine their impact in the early days!