This story is really more of a (very) short essay on the nature of reading fiction and how it becomes part of our lives. To Leigh, fiction stories are like parasites, sliding into our brains through letters and words, and, ultimately, changing who we are. Now, I don't mean that who we are is what we read, and I don't think that Leigh believes this either. More along the lines that what we read affects how we think, that, much like a parasite affects the way a body works, fiction has an effect on how we think. Fiction, especially good fiction, can take us to metaphorical places we've never been and prompt us to have thoughts we've never had before.
Really, none of this is a new idea. Even the earliest works of fiction, like Beowulf or the Iliad, aren't just telling us a story. They're prompting us to think in a certain way about that story. The author(s) of Beowulf wants us to think about what makes a great hero and what makes a great king, for instance.
What is new about Leigh's story how he takes the idea of fiction infiltrating your brain for it's own purposes and turns it into something almost malicious, and certainly creepily insidious, by having the story talk directly to the reader. The directness doesn't quite always work for me, but Leigh also allows for that in the story as well, and works in some interesting safeguards so as to not lose "you." He also keeps the work very short (three and a half pages), which I think was a smart move, because, other than the central hook of the story, there isn't a lot to work with here.