In some versions, the reason the dead rise and attack is unknown, rendering the whole situation inexplicable. In others versions, a specific parasite or infection is known to be the cause, framing events much like a plague. Some stories have every corpse rise, regardless of the cause of death, whereas others require exposure to the vector of infection.
In either scenario, this causes the outbreak to become an exponentially growing crisis: the spreading “zombie plague” swamps law enforcement organizations, the military and health care services, leading to the panicked collapse of civil society until only isolated pockets of survivors remain. Basic services such as piped water supplies and electrical power shut down, mainstream mass media cease broadcasting, and the national government of affected countries collapses or goes into hiding.