I read the book like 4 years ago, so I don't quite remember, but I think it measles, and you can cure that with a vaccine... I hope I'm right, very sorry if im not.
This disease refers to the disease of prejudice or discrimination, especially referring to the blacks since Maycomb was a racist place. Atticus was reminding Scout to not be influenced and led astray by their environment.
I read this book 5 years ago, so there may be more to explain, but this is the gist of it that I remember.
