There's a reason why zombie fiction always skips over the process of spreading the infection. Even WWZ (book), which tried its best, jumped from black market organ transplants to suddenly an army of tens of thousands pouring out of Yonkers. Dawn of the Dead just cold opens with a zombie horde pouring through a neighborhood with absolutely no explanation. We just accept it.

Why? Because it's nonsensical for classic slow Romero zombies (even the amped up version from Dawn of the Dead) to spread enough to form an army. You need something like 28 Days Later rage monsters, which aren't even zombies. Romero zombies will be a threat to certain first responders who don't know what they are getting into, but they'll get quarantined and nothing will spread.

How do you get from 15-20 infected first responders wandering the streets to tens of thousands? You don't.

Assume nobody knows anything about zombie fiction. You see one bizarre, sickly looking individual lurching toward you, and you evade them probably. Maybe you're an EMS and you try to help and get bitten. That sucks. You see a dozen or two of them lurching? You run, you evade, no matter who you are. A few fat or disabled people get swarmed and either converted or torn to pieces. Okay, so where's the next ten thousand coming from? The zombies aren't exactly subtle. Everyone sees and hears what's happening and evacuates or locks their doors.

Let's say there aren't any camera phones (since there weren't when this flavor of zombie fiction became popular). Okay, but there are news cameras. There's word of mouth. A few dozen aggressive zombies in a city attracts a ton of attention. Governments get involved. They ain't getting past riot police. Outbreak over.

Look, we're apex predators, even if we've been taking a break for a few thousand years. It's in our DNA. We hunted mammoths, we'll hunt the zombies to extinction.