This is a very intelligent question. When you speak of rabies, you are referring to the mad dog that was walking around the neighborhood. When Calpurnia sees it, she phones Atticus. Atticus and Heck come to take care of it. In the end, Heck asks Atticus to take the shot, because he is the best shot in town. In fact, when Atticus was younger, he was called one-shot Finch. Atticus finally puts the dog down.
This is an important part of the story, because it shows that there is madness (represented in the rabies) in Maycomb and people like Atticus have to do something about it. The madness shows in its blind racism. The trial of Tom Robinson is an example par excellence. Who will stand up for Tom? People like Atticus will.
In a similar case, we see madness (or rabies) in the mob that came to hurt Tom before the trial. They were even going to hurt Atticus, who was watching over Tom. The mob finally came to its senses when they saw Scout. The language Atticus uses reminds us of the dog scene. Here is the quote:
“So it took an eight-year-old child to bring ‘em to their senses, didn’t it?” said Atticus. “That proves something—that a gang of wild animals can be stopped, simply because they’re still human. Hmp, maybe we need a police force of children... you children last night made Walter Cunningham stand in my shoes for a minute. That was enough.”