Well the standard idea for zombie stories goes like this:
The virus or cause of the zombie is infecting the dead, IE it only affects things that are dead but does not mean it's not in living things.
Zombie bites may not quite spread the infection itself in this case because it's in everybody, but instead they cause a rapid and dangerous infection that kills much quicker by causing it to lead to an infection in the heart or brain, let's say. It's a similar effect to how the komodo dragon kills.
This again is assuming we're talking Day of the Dead style, where you become a zombie simply by dying. I wouldn't know, I don't watch Walking Dead.
As for why it just now activated, well viruses can mutate very rapidly. It's why we can't cure the common cold or flu yet.
The virus or cause of the zombie is infecting the dead, IE it only affects things that are dead but does not mean it's not in living things.
Zombie bites may not quite spread the infection itself in this case because it's in everybody, but instead they cause a rapid and dangerous infection that kills much quicker by causing it to lead to an infection in the heart or brain, let's say. It's a similar effect to how the komodo dragon kills.
This again is assuming we're talking Day of the Dead style, where you become a zombie simply by dying. I wouldn't know, I don't watch Walking Dead.
As for why it just now activated, well viruses can mutate very rapidly. It's why we can't cure the common cold or flu yet.