The Sickness of Timothy Frisby
In Chapter 1 the book explains that the Frisbys live in a slightly damaged cinder block. Which has a little crack in it for the mice to get in and out of. But then Mrs. Frisby's son fell sick with pneumonia. Mrs. Frisby realized that he was sick when he told her that it hurt too much to get out of bed. Then she went to fetch Mr. Ages.