At the nursing home, Abe and Edna had struck up a romantic relationship. Since both of them were in their eighties, their physical contact was rather limited. However, every evening as they sat together on the sofa, Edna would unzip Abe's fly, pull out his penis and hold it in her hand for twenty minutes. This satisfied the two of them adequately.
One day, Abe told Edna it was all off. He told her he was leaving her for Mabel - one of the other old dears at the nursing home.
Naturally, Edna was a little miffed. "Heavens! What's she got that I haven't got?" she asks.
"Parkinson's," said Abe.