Maycomb's usual disease is racism and prejudice. Atticus coined the phrase, and he hopes that Jem and Scout will come to him with their questions and follow his lead.
"Right. But do you think I could face my children otherwise? 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… 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?"
To Kill a Mockingbird