The Vestal Virgin was the symbol of the city, specially set apart in order to incarnate the impregnable boundaries of Rome. When Rome was subject to violence, it was because she had been violated. Yet it was this very status that made it possible for her to be used as a witch figure whose sacrifice averted the anger of the gods. She could become a pharmakos. Like the pharmakos, she was a ritually pure victim. Seneca (Cont. 4.2) explicitly compared the physical perfection of the sacrificing priest to the physical perfection of the sacrificial victim.
Yet we hear of no examination to determine a loss of virginity, apart from the trial by ordeal of burial alive. To have definite medical evidence one way or the other would destroy that precarious balance that Girard points out, since the victim must be simultaneously pure and yet guilty. Like the pharmakos, she was paraded through the town in order “to absorb all the noxious influences that may be abroad.” She partook, therefore, of the dual nature of the pharmakos, even as pharmakon has a dual sense. The ritual victim is both disease and cure. Dion. Hal. 9.40.1 (on the murder of Urbina in 472) makes the mechanism clear: once the Vestal was buried alive, the plague that had afflicted the women with sterility and miscarriages ceased (again, note the standard association of witchcraft with plague).
Holt N. Parker, “Why Were the Vestals Virgins? Or the Chastity of Women and the Safety of the Roman State” (2004)