In chapter 9 of To Kill a Mockingbird, what is Maycomb's "usual disease"?
In chapter 9, Scout overhears Atticus having a serious conversation with his brother about the trial. When Uncle Jack asks Atticus how bad it's going to be, Atticus tells him that it essentially boils down to Tom Robinson's word against the Ewell family's testimony. Atticus explains to Uncle Jack that the jury cannot possibly take a black man's word over a white man's account, but he intends to "jar the jury a bit" before the case is over. When Uncle Jack suggests that Atticus not defend Tom, he responds by saying,
"You know what’s going to happen as well 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" (Lee, 91).
The "usual disease" that Atticus refers to is Maycomb's prejudice and racism. Atticus mentions that the white people of Maycomb go "stark raving mad" when race relations are brought up. Atticus recognizes how prejudiced much of the town is. The racist citizens of Maycomb criticize Atticus for supporting a black man and also attempt to lynch Tom Robinson. He fears that his children will imitate their racist neighbors and become prejudiced towards black people. He also hopes that they'll survive the trial without bitterness and avoid prejudice of any kind in its aftermath.
When Atticus says this, he is talking to Uncle Jack about the upcoming trial of Tom Robinson, and expressing his fears of how the trial might impact Jem and Scout. He is worried that Scout will let her temper get the better of her, and that she will try to fight anyone who brings it up; he indirectly warns her against this and asks her to refrain (she is listening behind the door and he knows). Then, he says that he hopes that he 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." So, the "usual disease" that Atticus is referring to is how people do horrible things because of racism. Take for example the mob that gathers outside the jailhouse; otherwise decent, law-abiding citizens are overrun by fear and racism, and it prompts them to violence and evil. This disease-racism infects their blood and makes them behave crazily, just like some real, actual physical diseases can. So Atticus hopes that Jem and Scout get through the trial without catching the infectious hatred that will be spewed, and that they will "come to [him] for answers instead of listening to the town."
The usual disease is the mindset that the others have set up about racism. Most of the people in Maycomb believe that it is okay to be racist and that Tom deserves to be in jail even though he is innocent. Atticus is worried that Scout will get angry and get into a fight with people who are racist and do not look at the facts.