The virus infects everyone, but doesn't manifest itself until you die. No matter how you die, you've already got it and will reanimate unless your brain is destroyed. It doesn't spread by the Zombie's bite.
Just my guess.
I think your guess is on target.
Based on various tidbits of information in TWD, including the revelation from the CDC researcher (as stated by Rick) that every one is "infected", I'd lean towards it being biological in nature vs Cosmic Rays, Alien intervention, CO2 levels, ect.
The last two have real world examples IIRC, infecting insects and small critters and either controlling or influencing them into actions and behaviors that are not to the hosts benefit. Mind you, when the infected critters die, they stay dead. They're "zombie like" when infected, not dead. Eviscerating one of them would kill it, not leave the disemboweled corpse moving about.
Whatever it is, FTWD seems to indicate that whatever "it" is seems to be passed around like the flu. Flu contagion rates can be very very high and if the infection doesn't cause corpses to reanimate until there has been a sufficient build up in the host body of the infectious organism(s) then onset of reanimated dead might be long enough out from the time "it" first started to spread in the GP that the overwhelming majority of the population had already been exposed (whether symptomatic or asymptomatic re: "flu like" symptoms) before the first walking corpse started moving about and causing a ruckus.
A zombie's bite doesn't cause you to turn, it just kills you. Either from blood loss/trauma or from infection. For real world examples look at the Komodo Dragons. Their bite is fatal due to the sewer of bacteria and viruses in the foamy, ropey drool in their filthy mouths. IIRC death is within 3 days or so for deer after being bitten, That kindo of matches up with TWD's presentation of time between being bitten and dying/turning. Those already ill or weakened succumb to the infection sooner, those that are more robust may fight it a bit longer.
“Political tags – such as royalist, communist, democrat, populist, fascist, liberal, conservative, and so forth – are never basic criteria. The human race divides politically into those who want people to be controlled and those who have no such desire.” Robert A. Heinlein