TIL the largest cemetery in the world is Wadi-us-Salaam in Iraq. It has 6 million bodies, covers 6.01 km2/2.32 sq mi of land and has been in use for at least 1400 years.
I've always thought of zombies as an infection of the living or recently living. Graveyards are more of an undead thing, you need necromancy to worry about they coming back to life.
Who knows? Depends on the zombie. Before Night of the Living Dead, they were all supposedly raised from the dead by voodoo, which I imagine a lot of Haitians found pretty fucking offensive, but NOTLD was already in the '60s.
Also, I may be misremembering, but I'm pretty sure one of the Return of the Living Dead movies (not to be confused with Night of the Living Dead movies) featured zombies coming out of graves.