Ghosts are everywhere! Dead sims keep calling their relatives. At the hospital all co-workers are only ghosts. When my sims go to bar or nightclub there are only ghosts. Why is that happening??? :(
You must have had a lot of people pass away in your game, and/or if you have GT, you've run into a lot of Ghost Nights at the bars/clubs. If you want to get rid of the ghosts completely, go into Manage Households which is at the top right in map-view and under "Unplayed Households" you'll most likely find a bunch of ghosts that you can delete.