To Kill a Mockingbird is slow in discussing the racial dynamics that come to the forefront in the trial of Tom Robinson. There are a few casual references to African Americans by Scout and her friends in the first few chapters, but no direct engagement with racism until chapter nine, when Scout is upset by Cecil Jacobs, one of her schoolmatesm who tells everyone in the schoolyard that Atticus "defends n----rs." This angers Scout to the point that she is ready to fight the boy.
As she discusses the matter with Atticus, he reveals that he is defending Tom Robinson, and the issue emerges again at the Finch Christmas Eve party, when Francis essentially says the same thing in an effort (which turns out to be successful) to provoke Scout.
After this incident, Atticus alludes to the racism in the town in conversation with Jack. He says that the trial boils down to a black man's word against the word of the Ewell family, who are white. He goes on to refer to racism as "Maycomb's usual disease" and says that "why reasonable people go stark raving mad when anything involving a Negro comes up, is something I don't pretend to understand."
This foreshadows much of what will come in the rest of the novel. After the early portrayal of Maycomb as a fairly typical, if quirky Southern small town, we learn that the town is typical of the 1930s South in another way—it is infected with the "disease" of racism.