Why would everyone have the virus? Not everyone would have it because you still have to obtain the disease from an infected source. Someone who catches Hepatis A (one of the most contageous known diseases) in Atlanta is not going to mean everyone in Savannah is now infected. This is in reference to the dual pathogen theory so there would still need to be a vector of transmittal. Its also a hell of a lucky coincidence. Because if these two pathogens didn't happen to become known at exactly the same time you would have dead people turning into zombies but their bites would be no worse (maybe a little worse due to the decaying state of the biter) than a bite from a living person. Any pathogens they transmit would be those already found today (mostly treatable) and not as lethal. If the disease was just "magically" spread by something that everyone is exposed to (say radiation from the sun) then we are presented with the interesting scenario where anyone who has died previously and whose brain has not decayed beyond some point of non-use would be reanimated in their graves, mausoleums, morgues, etc. Although if you take the starving idea presented later in TWD . . . they might not be able to do anything??? The movie "Fido" used radiation from outer space as the cause of zombieism so everyone who died would become a zombie. If Grampa keels over from a heart attack . . . lookout!!!