Marley's ghost: just indigestion?

"Why do you doubt your senses?"

"Because," said Scrooge, "a little thing affects them. A slight disorder of the stomach makes them cheats. You may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato. There's more of gravy than of grave about you, whatever you are!"

Scrooge was not much in the habit of cracking jokes, nor did he feel, in his heart, by any means wagging then. The truth is, that he tried to be smart, as a means of distracting his own attention, and keeping down his terror; for the spectre's voice disturbed the very marrow in his bones.

Charles Dickens, A Christmas Carol (1843), Stave I