Lore explanation for spontaneous corpse reanimation

I haven't watched the show or read the comics, so I don't know if they specify the exact nature of zombies in TWD universe. But in most zombie stories I've read a corpse will only reanimate as a zombie if they've been bitten, not if they die from other causes such as gunshot. In S&S however any newly killed NPC will turn into a zombie as long as they have an intact and attached head. I understand and appreciate it as a gameplay mechanism, whether a freshly killed enemy gets up again for a second crack at you, or if you tactically kill a guard to make a zombie that will sow chaos in the enemy ranks.

In terms of lore, my random headcanon is that all the survivors have a minute amount of zombie virus in their system, from surface contacts, inhalation, and just living in close proximity to zombies for a long time (sidenote: does anyone know how long since the apocalyse this game takes place? Seems like 6-12 months).

A living person's immune system is enough to counteract this virus, but once they die (of any cause) the virus is able to take control of the body and turn them into a zombie until the brain is destroyed.

Is this a sound theory? Does this bother anyone else, and if so you do have a better explanation?