A certain corporation tried to create a resurrection drug but made a zombification drug instead, which they illegally dumped into the ocean, spreading it across the world.

A certain religious organization tried to perform a miracle by developing a zombie virus, then carried out terrorist attacks in various locations using the virus, resulting in many casualties.

A certain research institute was breeding organisms that parasitize humans for experiments, but they escaped and soon infested the entire world.

In short, the world became infested with zombies, just like in the movies.