While exploring,, I'm being attacked by a ghost. Receiving bleeding damage from an unknown and invisible threat. Knocked unconscious and killed after taking approximately 10 hits.

No sound effect (gunshots/zombies/footsteps etc), just player grunt and movement from hit. Exactly the same experience as being melee attacked by zombie or player but without any sound or visual enemy.

It has happened twice in urban areas (near zombies) and once in a rural area. Official EU server.