It Follows
The main character, Jay, is stuck with a parasitic "ghost" that slowly follows her around in an attempt to kill her. Only Jay can see it, and she has to give it to someone else by sleeping with them. The ghost takes on many different forms. Jay gave it to her friend, Paul, who gave it to a prostitute. The director opened it for interpretation, but most people agree that the woman that Paul gave the ghost to died, and now it's following Paul yet again.