To Mrs. Ansley "Roman fever" conjures the romantic memory of a night of intense life-changing passion that gave her Barbara, and the sentiment of treachery that she deals to her old rival.
The title of Edith Wharton's short story is a double entendre as its meaning sugggests both an illness that one can incur if in the Roman night air of the Forum, and the idea of intense passion. In the second visit to Rome as Mrs. Slade and Mrs. Ansley accompany their daughters to the infamous city, the history of the fever of Roman treachery is parodied in the two dowagers who have been "intimate since childhood."
The conversation between the two old friends who "visualized each other...through the wrong end of her little telescope" reveals the levels of treachery that they deal to each other. First, Alida Slade discloses that it was she who wrote the passionate letter of invitation from Delphin, her fiance at the time, to meet at the "danger hour," a rendez-vous that apparently led to Grace Ansley's having become ill. But, Grace finally discloses that she responded to this letter, and so she was not alone that night. Furthermore, after Alida Slade boasts that it was she who married Delphin and Grace who "had nothing but that one letter that he didn't write," Mrs. Ansley delivers the coup de grace [the final blow]: "I had Barbara."