In Chapter 9 of "To Kill a Mockingbird," Atticus talks with his brother Jack about the forthcoming trial of Tom Robinson who is being accused of raping Mayella Ewell.. Atticus explains to Jack that the jury "couldn't possibly be expected to take Tom Robinson's word against the Ewells'." Adding to this, Atticus explains that he has hoped that he could get through life without being involved in such a case, but he has been appointed by the court. Jack comments, "Let this cup pass from you, eh?"
Right. But do you think I could face my children otherwise? You know what's going to happen as as I do, Jack, and I hope and pray I can get Jem and Scout through it without bitterness, and most of all, without catching Maycomb's usual disease. Why reasonable people go stark raving mad when anything involving a Negro comes up, is something I don't pretend to understand...I just hope that Jem and Scout come to me for their answers instead of listening to the town. I hope they trust me enough...Jean Louise?
Scout writes that it is many years before she realizes that Atticus means for her to hear everything he says so that she will understand the disease of social/racial prejudice. The metaphor "Maycomb's usual disease" is part of Harper Lee's social commentary.