In folklore, an Ondine is a female water nymph who can acquire a soul by marrying a man and bearing a child. The legend is the subject of many works of fiction. In some versions of the myth, Ondine curses an unfaithful partner to stop breathing if he ever falls asleep. This legend has also lend its name to “Ondine’s curse”, a medical condition called Congenital Central Hypoventilation Syndrome, where the patient loses control of breathing, thereby placing them at greatest risk when asleep.