The Two Doctors Stone - S5.E26

Alex and Donna prepare to go away for the weekend, but Donna suspects Trisha will be sick. Donna sees Trisha eating bananas and she always saw Jeff eating bananas before he became ill. Alex's collegue Dave (Bob Crane) tries to convince them otherwise ... and it turns out that Jeff is the one who came down with a fever. Trisha had been eating bananas because of her promised treehouse and she had wanted to climb trees like a monkey.