Mrs. Smith was a hypochondriac.  Dr. Jones was fed up with her constant complaints about non-existent illnesses, so he started palming her off with a mild sedative to keep her happy.

One day she complained about chest pains and the doctor prescribed his usual treatment.  This time however, the pain was real and Mrs. Smith died of a heart attack.

On hearing of her death, Dr. Jones was so upset he died of shock.  Mrs. Smith and Dr. Jones were buried next to each other in the cemetery.

The next morning, Dr. Jones heard a tapping on his coffin, followed by a voice saying, "Dr. Jones, this is Mrs. Smith. Do you have anything for worms?"