I like it when Zombie Stories have a secondary crisis which helps justify why Militaries and Governments are overwhelmed.
Zombies alone can eventually be conquered by competent societies that know how to fight the infection but that’s not easy when there’s a civil war or environmental disaster ongoing at the same time.
However, this isn’t necessary if the Zombies alone are so deadly and efficient at spreading without another global issue indirectly aiding them by distracting authorities.
Left 4 Dead is a prime example:
”This virus defies anything we’ve ever seen. Sometimes it’s airborne. Sometimes it’s not.”
”It mutates daily. We’re trying to cure it and we can’t even pin it down.”
THE DOCTOR, THE SACRIFICE COMIC