What disease common in Macomb does Atticus hope the children never contact?

That would be racism.

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.