The most crucial to a reader's understanding of an expository text would be how effective the text is written to convey a clear and concise idea to the reader. It should be able to penetrate a reader's comprehension well and establish a clear way to get the point across.
The irony is that they are smiling with “torn and bleeding hearts”.
The idea it emphasizes is that they are smiling to hide their pain.
The overall message revealed by the irony is that they are in considerable emotional turmoil and faking happiness to mask their distress.
Answer:
An epidemic of fever sweeps through the streets of 1793 Philadelphia in this novel from Laurie Halse Anderson where "the plot rages like the epidemic itself" (The New York Times Book Review).
During the summer of 1793, Mattie Cook lives above the family coffee shop with her widowed mother and grandfather. Mattie spends her days avoiding chores and making plans to turn the family business into the finest Philadelphia has ever seen. But then the fever breaks out.
Disease sweeps the streets, destroying everything in its path and turning Mattie's world upside down. At her feverish mother's insistence, Mattie flees the city with her grandfather. But she soon discovers that the sickness is everywhere, and Mattie must learn quickly how to survive in a city turned frantic with disease.
