OK so this is what I am thinking.
Everyone already has the virus. All are infected.
However as long as you are alive the virus is kept in check in the human body by their own immune system. you are fine, even normal.
Who knows maybe the virus even keeps you healthier. Like it kills cancer cells or something.
However due to the aggressive yet beneficial nature of the virus the balance is a pretty even balance.
Like a 1.2 to 1 cellular ratio. Your white blood cells are basically doing all they can just to keep gauges on “even keel”.
Obviously the majority of the virus resides in the brain pan. Maybe it even acts as a collective like bees.
However once you die the virus says “nu-uh I ain’t done here! I am in charge now!” and so off you go.
The virus is now hunting down the raw materials it needs to keep going. Well you cant get a blood transfusion from a cow so you know it needs genetically compatible materials. Since it is a virus it doesnt know about upkeep and taking care of your toys so it just runs around pushing the vehicle till it just cant move no more.
So now humans are blue plate specials.
Since the host is dead already the virus is now allowed to run rampant and build up in the hosts system to an extremely toxic level.
So when you get bit or other fluids transpose between hosts the balanced scales are tipped. The walkers virus goes “WHOOHOO” and jumps ship like any good virus will do.
Hey they just wanna watch the world burn too.
Since YOUR virus just had a crapload of relatives showup for the holidays your immune system goes “oh balls. I quit.” and down you go into zombieland.