"My folks said your daddy was a disgrace an' that nigger oughta hand from the water-tank!" (page 102)
"YOU CAN JUST take that back, boy!" (page 99)
The "disease" Atticus is talking about, is racism. Talking could lead to contracting this disease because the kids could hear other people's views, and start to think the way they were thinking. This excerpt could be telling the reader that later on in the book, Scout, and/or Jem start to have different views on different people and that they catch "Maycomb's usual disease".
"He's nothin' but a nigger-lover!" (page 110)
"I don't know what you're talkin' about, but you better cut it out this red hot minute!" (page 110)