Fanfiction for TV shows and movies stuck around in obscure paper-based form called fanzines for a few decades before it found its true home on the Internet, where it has since exploded, with millions of new installments every year. It's easy to assume the members of this subculture are just lonely, obsessed kids in their parents' basements or even lonelier, equally obsessed middle-aged women, but the truth is, you've been reading (maybe even enjoying) fanfiction for a long time. And we're not just talking about adding zombies to Jane Austen.
Jane Austen fanfiction first appeared around 1850, and fan stories based on the works of Arthur Conan Doyle and Lewis Carroll not much later. A Sherlock Holmes fanfiction-writing group, the Baker Street Irregulars, started in 1934 and has since had members such as Isaac Asimov and Franklin Delano Roosevelt.
And even these guys were way late to the fanfiction party. Fifty years after the first complete narrative of the semi-mythical King Arthur appeared in a book called The History of the Kings of Britain in 1138, another writer, Chretien de Troyes, rewrote the story and added in his own character: Lancelot, the handsome knight who steals Arthur's love, Guinevere. He also put in a story about a random cup that he called a "grail." Thirty years after that, another guy decided it would be cooler if it was the Holy Grail. Since then, King Arthur has been interpreted by everyone from Mark Twain to C.S. Lewis, and rewrites are still going on 800 years later.