In the book, the end of the chapter where Neville finds the dog. After spending a month trying to get the last living creature on earth other than himself to allow Neville to pet him, it finally looks like he will have a companion. It'll be easier just to read it yourself:
"You'll be all better soon," he whispered. "Real soon."
The dog looked up at him with its dulled, sick eyes and then its tongue faltered out and licked roughly and moistly across the palm of Neville's hand.
Something broke in Neville's throat. He sat there silently while tears ran slowly down his cheeks.
In a week the dog was dead.