It is common to assume that ghosts are the spiritual remains of those who are unwilling or unable to move on. Exactly where this moving on is supposed to go is unknown and in dispute, but the general assumption that some portion of the dead do not undertake the journey is widely held. The dead are supposed to go somewhere, but ghosts for whatever reason do not.
A less examined assumption about ghosts is that they are the spiritual remains of a person, whole and entire. It stands to reason that this assumption is not to be taken for granted, and that ghosts in some sense are what the dead left behind when they left. The old adage that you can’t take it with you comes back to haunt us, as it were. There is no reason to assume that the afterlife requires each and every aspect of our mortal countenance brought along. Shedding excess mortality would, when seen in this light, be a perfectly reasonable thing to do.
This opens up for the possibility of unrelenting ghosts of uncured toothaches haunting unsuspecting and unlucky survivors. Which does, to be sure, reframe the common wisdom that prevention is better than cure.