Some sort of family fairing in the some sort of futuristic "dark ages", where there is a massive plague that eliminates nearly everyone, and the family is one of the very few of what once was America, if you like spooky and sad stuff, like say everyone in narrator's family dies. At the end of the book, he also closes his eyes for the last time.
You should totally do it in the form of a diary, and leave it up to the reader to think if the guy just lost the book after the military arrives to inject the "cure" into him during the night after he put his diary down, because near the end he contracted the disease, or died in his sleep.