No-one knows where the vile corpses of undead came from, in-fact, I was one of the few that did. The survivors had discovered that they were a nasty side-effect of an immortality spell, which destroyed its caster Lord Voldemort, before it was completed: expelling raw energy and forcing corpses to rise.
Now, before we get any confusion, these corpses were not the demonic dark creatures known as Inferi, these were what muggles had named 'zombies', simple brain eating corpses.
That were immune to magic.