Much to Mattie's annoyance, she and her mother have been invited to afternoon tea by Pernilla Ogilvie. Mattie's mom is positively thrilled by the news; in fact, she says it's the best news in weeks. But given that Philadelphia is in the middle of a deadly outbreak of yellow fever, that's not saying much.
In any case, Mattie's mom is really looking forward to breaking bread with Pernilla Ogilvie, not least because the Ogilvies are such a prominent family in town, the kind of people from whom one doesn't normally receive an invitation to afternoon tea.
As we've seen, Mattie is not quite so enthusiastic about the invitation. She regards Mrs. Ogilvie's daughters as frightful snobs. But her mother's not going to pass up such an opportunity to hobnob with the great and the good, and so Mattie's going to afternoon tea at the Ogilvies' whether she likes it or not.
As it turns out, afternoon tea does indeed turn out to be an unpleasant experience as Mattie suspected it would be—but not for the reasons she originally envisaged. One of the Ogilvie girls, Colette, grasps the coffee table and pulls herself to her feet, knocking over a pitcher of cream. With all eyes upon her, Colette starts panting heavily. It's patently obvious that there's something wrong.
Colette then starts whispering that she's burning, and, in a matter of seconds, she's collapsed to the floor. Mrs. Ogilvie bends down, and placing the back of her hand against her stricken daughter's forehead, immediately knows what's wrong: Colette has been struck down by yellow fever.