Tiber River with Tiber island in the background
Legend has it that in the 3rd century B.C., during a plague, the Romans went by ship to Epidaurus, in Greece, to learn from the god Aesculapius how to escape the scourge. But when the returning ship was ascending the river, the god's sacred serpent slipped out of it, at the point where the island was, indicating that that island was to be consecrated to him.