Sure thing! But a warning, these are major spoilers for anyone who wants to play the game.
Major Secret World Spoilers
The zombies are animated by a great amorphous living disease called “the Filth”. It’s a black, tar-like oily sludge. The Filth comes from “the Dreamers”, cosmic horrors akin to something from the Cthulhu pantheon. The Dreamers want to escape their metaphysical prison and their dreams of escape manifest in the real world as the Filth.
The Filth is sentient and can mutate people into monsters if they touch it, and even being in proximity to it for too long causes insanity from its psychic whispers. Many of the monsters in the game come from it (but not even close to all of them).
In the New England town of Kingsmouth (think Stephen King + Innsmouth) on Solomon Island, a fog has surrounded the area, isolating it from the outside world. The fog has turned people into zombies. The fog is a product of Draug, ancient Norse horrors from mythology. The Draug control the fog and the zombies, and are themselves empowered by and influenced by the Filth.
The Draug came to the island from the sea, and are mostly encountered on the beaches or just offshore from the island as you explore.
It’s a different spin from the usual trope of zombies either being a product of some government bioweapon broken loose or some infernal curse. Instead it’s from old Norse legends but tied into the overall Lovecraftian lore of the game. Kingsmouth is just the starting area of the game, and the lore I gave here is a tiny fraction of the huge story you uncover through the game.