Really good horror film that is creepy and dreamlike and based in a reality similar to our own, but maybe..slightly to the left. In general it's a film about Jay, a high school girl whose boyfriend purposefully infects her with a unique "disease". Until she passes the "disease" along to someone else, she is doomed to be slowly followed by a random person that only she can see. This person can take the form of anyone--people she knows, and total strangers--and if it catches her, she's certainly going to be killed by it. If she makes sexual contact with someone else, she can pass it along, but the resultant death of the person she passes to will result in it coming back to haunt her. (As Jay unfortunately has confirmed to her upon a friend's death, who doubted the veracity of her story and screwed her.) Multiple attempts are made on the thing's life, but it can't be killed by normal means, whatever it is.
What is "it", exactly? We aren't entirely certain. Could be a metaphor for venereal disease, HIV/AIDS, the horror and uncertainty of adolescent sexuality or intimacy, or any other number of things. The writer and director, David Robert Mitchell, says that it's not something he's interested in confirming because it's not something that can be narratively confirmed. Jay and the other characters are just living in a nightmare, and nightmares can pick at anxieties without having to make sense at the highest level. It certainly feels like it's based in a dream-like setting, because it takes place in a world that feels similar to our own but doesn't have modern cell phone technology in a recognizable way. The entire purpose of the movie is to feel "off", I think, and it really works. It's also genuinely a scary movie because it is more or less like watching an extended anxiety dream resistant to logical interpretation but still filled with vast amounts of id lurking beneath the murky surface. (Also scary because I actually have a number of recurring nightmares about being followed, which is part of what inspired Mitchell in the first place.)