The virus is airborne. I believe everyone's infected, just some have immunity.
Also, it was pretty evident Shane was getting offed - he's in Darabont's latest show next year....
Yes, everyone is infected.
You don't reanimate if there is damage to your brain.
People who die of non-bites or scratches reanimate quickly like Shane.
People who are bitten or scratched get a massive infection similar to being bit by a Komodo dragon which has a mouth full of septic pathogens. Hence, you have an Amy or Jim type agonizing (high fever, bleed out from a bite, etc) death and reanimate.
I love how people think the writers are making up it as the go along.