Bug is a 2006 psychological horror film directed by William Friedkin and written by Tracy Letts, and starring Ashley Judd, Michael Shannon, Lynn Collins, Brían F. O'Byrne, and Harry Connick Jr. Letts adapted the screenplay from his 1996 play of the same name. It follows an isolated woman living in a rural Oklahoma motel who spirals into psychosis after taking in a mysterious Gulf War veteran.

Bug debuted at the 2006 Cannes Film Festival before being purchased by Lionsgate, which released the film the following year in May 2007. The film received generally positive reviews from critics, who praised its intensity, directing, acting, and take on paranoia, but were polarized about its writing, in particular the film's ending. Friedkin and Letts collaborated again as director and writer on the 2011 film Killer Joe.

Waitress Agnes White works at a lesbian bar while living in a run-down motel in rural Oklahoma. Unable to move on after the disappearance of her young son Lloyd nearly ten years before, Agnes binges on drugs and alcohol with her lesbian friend R.C.

Agnes is plagued by silent telephone calls that she believes are being made by her abusive ex-husband Jerry Goss, who has recently been paroled. Jerry had attempted to murder her, and stalked her prior to his arrest. One night, R.C. introduces Agnes to drifter Peter Evans. Agnes strikes up a bond with Peter and allows him to stay the night. The pair's sleep is interrupted by a malfunctioning smoke alarm, which Peter destroys with a Magic 8 Ball handed to him by Agnes.

The next morning, Jerry barges into Agnes's room while Peter is out getting food. After menacing her, Jerry attempts to reconcile. Agnes then insults him, and Jerry punches her. Jerry leaves, promising to return. Peter later comforts Agnes, and the two have sex that night.

Peter wakes Agnes up in the middle of the night, having apparently found an aphid in the bed. A paranoid Peter kills many aphids, and reveals that he is being pursued. However, he refuses to reveal who is hunting him on the grounds that it will put Agnes in danger and flees. Agnes, feeling abandoned, bursts into tears and a remorseful Peter returns. Peter reveals himself to be a veteran of the Gulf War. He claims that he was subjected to biological testing by the U.S. government, that the anonymous calls were made by government agents in anticipation of his arrival, and that the room has become infested by bugs planted by the government as part of the experiments. Peter's movements and behavior become more erratic as he fights the "invisible" bugs he claims are infesting his body.

Over time, Agnes begins to share in his behavior. R.C., who has taken Agnes to visit a dermatologist, is convinced that there are no bugs in the motel. Agnes has also reported to the motel owner that their room has bugs but says that he told her that none of the other rooms have a bug problem. R.C. tries to convince Agnes to leave Peter, mentioning that someone named Dr. Sweet is looking for him. But, after Peter has an episode, Agnes slaps R.C. and tells her to leave. Peter and Agnes isolate themselves, closing the room, covering it with flypaper and aluminum foil to fend off communications, and lighting it with the glow from bug zappers. Peter, believing that microscopic bug egg-sacs were implanted in one of his teeth, tears the tooth out with pliers. After examining the tooth using a child's microscope, he believes that he sees the bugs, as does Agnes. Peter is progressively more obsessed with using the microscope, believing that the bugs are living in and eating his blood.

Jerry eventually bursts in with Dr. Sweet. Sweet sends Jerry outside and tells Agnes that Peter escaped from a mental institution where he was undergoing treatment and that delusions about insects are a known symptom of Peter's mental illness. Agnes distrusts Sweet, who later claims to know where Lloyd is and that the bugs are in fact real. Sweet attempts to convince her that he can help Agnes and Peter escape from the conspiracy.