See the problem is that we have melded to different genres into one and call it Zombies.
The split in genres are usually referred to as Romero Zombies.. and Boyle Zombies...
The problem is that the second one is a misnomer.
Zombies die and then reanimate to feed on the living. This is the very definition of Romero Zombies. (He created the cult classic Night of the Living Dead)
Boyle Zombies do not die therefore they aren't Zombies. They are "infected" with a virus and the transmission is via blood/biting. Boyle Zombies got their name from the creator of the Movie 28 Days Later. His name is Danny Boyle. In the Movie Boyle Zombies are infected with a weaponized Virus called Rage. It Speeds up the metabolism of the infected and drives them crazy and they go into a killing rage attacking people with their bare hands. Thus you get Fast Zombies.
Now here is the scary part. Romero Zombies have about a 1 in a billion chance of happening (Not my opinion there is actually scientific studies that lay out how such could occur and the chances.)
However, the chances of "Boyle Zombies" A.K.A. "infected" are far more likely. (still not a probable outcome)All you need is a nasty virus that attacks the brain (something like Rabies) which affects the higher order brain functions leaving the infected a walking/grunting drone that is subject to violent behavior. (Imagine Liberalism on steroids) If the transmission method mutates into a bugger that can get you by simple touch or body fluids transmission then you got yourself a full blown Zombiepocalypse brewing.
What were the odds of a big bang happening from nothingness?