"The Infestation Hypothesis" is the second episode in the fifth season of the American sitcom The Big Bang Theory. This episode first aired on Thursday, September 22, 2011 in a back-to-back with "The Skank Reflex Analysis".
An argument between Sheldon and Penny over a chair she found in the street leaves Amy caught in the middle of it, while Leonard tries to spice up his long-distance relationship with his girlfriend Priya.
Leonard is having a dinner date (or a "dinfast" date, as Sheldon calls it, due to it being 8 AM in Mumbai.) with Priya. To be out of his way Sheldon goes over to Penny's place to read his copy of the American Journal of Physics. With the way they act, Penny and Sheldon joke they are like an old married couple. He then asks his "wife" to serve a snack causing Penny to ask for a divorce. Sheldon complements Penny on the new chair she has acquired, saying it's very comfortable. She says she found it on the curb outside and she paid a homeless guy to help bring it up. Sheldon completely freaks out and takes his clothes off to take a shower, but runs back home to his shower when he finds a wet band-aid on the floor of Penny's.
At the university, Leonard, Howard, and Raj are using the $175,000 hydraulic thermoforming press to make panini, when Howard asks how the long-distance relationship with Priya is going. When Leonard mentions that they just talk, Howard tells him that they've got to do cyber-sex or Priya is just going to find "some fancy guy in a turban who grew up with Kama Sutra coloring books". (Raj finds that comment incredibly racist). Soon, Sheldon comes to find Leonard so he can check for "chair lice"; Leonard says no and thinks of it as nothing but another one of Sheldon's phobias.That evening, Sheldon goes to Penny's to check on the chair. She says she had it completely cleaned out, so he goes to sit on it. However, after he flicks a bug off his arm, loads of bugs start to crawl all over him. He suddenly snaps out of this dream at home in a way that makes Leonard jump, and he quickly goes over to Penny's to demand she remove the chair from the building. Every attempt he tries is unsuccessful as she refuses to move it. The last time he tries, Penny brings the chair's cushion to the door and he freaks out.
In bed, Leonard is chatting online with Priya, and is unsuccessfully trying to talk dirty. Priya decides to guide him with it and asks him to get naked. Just as she is taking her shirt off to reveal her breasts, the laptop screen freezes up, much to Leonard's dismay. Sheldon shouts from his room and suggests "resetting the TCP/IP stack" (sic), then asks to hurry it up as he is streaming a movie on Netflix.
The next day, Howard shows Leonard a machine with motion sensors that can be used to simulate kissing. He wants Leonard to test it but he does not want to kiss Howard. Raj tries it and he and Howard engage in French "internet kissing."
Later, while Amy is playing "The Girl from Ipanema" on her harp, Sheldon knocks on the door and after Amy let him in, he asks her to convince Penny to get rid of her chair. Amy wants Sheldon to kiss her somewhere he has never kissed her before. Sheldon thinks she is referring to a geographical location and offers to kiss her in Salt Lake City. Amy changes her mind and asks Sheldon if he wants to hear her play her harp. He declines, since he worries that harp music will cause him to have a flashback. Amy plays a note on her harp and Sheldon has a flashback just like he predicted.
Having been asked by Sheldon to talk Penny into getting rid of the chair, Amy is at Penny's place and comments on the chair. Penny suddenly realizes what is going on and is upset at Amy for doing Sheldon's work. She asks Amy to leave. Amy immediately apologizes and asks Penny to stay friends with her. To prove she is loyal to Penny, she sits on the chair, but finds something biting her "tushie". She gets up to find something moving inside the chair and the girls run out of the apartment screaming. Amy promises Penny to not tell Sheldon about this incident as long as she can tell her doctor, saying she will need shots.
Meanwhile, Leonard is starting to talk dirty to Priya on Skype again, until she informs him that her parents are with her and they stare at him sternly. Outside the building, Howard and Raj find Penny's discarded chair out on the curb. They decide to take it up to Leonard's and Sheldon's apartment so Raj doesn't have to sit on the floor anymore. As they are carrying the chair into the lobby, the camera focuses on the thing that is moving around in the back of the chair.
- Guest starring:
- Teleplay: Chuck Lorre, Jim Reynolds & Steve Holland
- Story: Bill Prady, Steven Molaro & Maria Ferrari
- The TV Critic: "Last episode I complained about the lack of Priya and credit to the writers she had not been forgotten. The long distance relationship is obviously doomed but at least we can get some mileage out of it while it lasts. .. The plots built around Sheldon's obsessions don't yield good results because they don't play to his comic strengths. Instead they tend to repeat the same points over and over again... One of my biggest beefs with The Big Bang Theory are that so many of its plots serve no purpose so obviously I wasn't thrilled with this. But in between the tedium there were some good moments." 
- The A.V. Club gave this episode a C
- IMDB user reviews
- Title Reference: Sheldon's phobia that Penny's chair is infested (which it turns out to be).
- Chuck Lorre's vanity card.
- This episode was watched by 14.94 million people with a rating of 5.1 (adults 18-49).
- This episode aired in Canada on September 22, 2011 with 4.053 million viewers and a weekly ranking of #2.
- In the United Kingdom, this episode aired on November 10, 2011 with 1.312 million total viewers and a weekly ranking of #7.
- In Australia, it aired on October 10, 2011 with 1.606 million viewers and a weekly ranking of #5.
- Episode transcript 
- Penny wears a yellow singlet top made by Sky (no longer available at shopsky.com - you'll have to hunt eBay/used retailers).
- Sheldon wears a teal and yellow striped 3/4 arm t-shirt and a blue with horizontal dotted stripes - both discontinued from Urban Outfitters.
- Bernadette does not appear in this episode (whether speaking role or background cameo). This is the first of two episodes not to feature Melissa Rauch in the fifth season; the second is "The Rhinitis Revelation" (S5E6). There is no mention of her or any on-screen appearance by her in this episode either.
- Amy plays the harp and knows two songs to it, "The Girl from Ipanema" and the Different Strokes theme song. Sheldon says he dislikes the sound of the harp due to its overuse in classic TV sitcoms; it makes him think that he will experience an episode from his past. (The moment Amy plays hers, Sheldon flinches and says "I'm sorry mommy, don't be mad at me".) This aversion, however, does not appear in episode 18-"The Werewolf Transformation" (S5E18) or the rest of the series.
- When Sheldon jumps and makes Leonard jump, he says it's like living with a Chihuahua.
- Leonard is terrible with cyber-sex.
- Like Leslie Winkle way back at the start of season 1, Howard and Raj use the university's resources for their own food purposes.
- Sheldon fixates on a crazy idea and takes it way out of proportion. Leonard mentions four times Sheldon has done this crazy thing: the time when he put GPS trackers in the garbage because he thought North Korean spies were stealing his doodles, when he thought chicken nuggets were human nuggets, the strangely shaped cloud that followed him around town and when he put on Leonard's shirt by mistake and thought he started growing again.
- Sheldon's suggestion of "resetting the TCP/IP stack" is tech-gibberish. The actual suggestion, "resetting the network connection", is devoid of the computer jargon that appeals to the filmmakers, although it is one that millions of smartphone owners used the date this episode aired.
- Amy seems aware that Sheldon may have Obsessive-Compulsive Disorder (OCD) as his ritualistic knocking behavior (Sheldon's Knock) is symptomatic of it. She also points out that he is in denial over it.
- Amy describes her and Penny as best friends, besties, BFFs, peas-in-a-pod, or sisters who would share traveling pants (referring to the movie, The Sisterhood of the Traveling Pants).
- This episode continues the axion calculation saga on the whiteboards began in "The Skank Reflex Analysis" (S5E1).
- First aired two days short of the show's fourth anniversary.
- The entire scene of Leonard and Priya having cybersex is removed.
- The episode goes from Penny scaring Sheldon with the cushion straight to Amy playing the harp in her apartment. This is the longest cut ever on the show, at three minutes.
- The cut scene is intact on the Complete Fifth Season DVD.
- Howard: So, how's it going with the long distance love affair?
- Leonard: It's not easy, but we're making it work.
- Howard: When you say "Making it work", does it include doing The Cyber Nasty?
- Leonard: What?
- Howard: You know, the virtual pickle tickle? The digital bow-chicka-bow-wow?
- Leonard: (Referring to his Skype date with Priya) You know, some people might say that it’s great that we’re trying to make things work long distance. They’d say things like, "Love is stronger than the miles between you."
- Sheldon: When I rise to power, those people will be sterilized.
- Penny: (After Sheldon's three successive triple-knocks at her door) What’s up, buttercup? … What’s the word, hummingbird? … What’s the gist, physicist?
- Leonard: (To Priya, attempting cybersex, after he removes his shirt) Tah-dah! Man nipples!
- (Amy is playing her harp to a Bossanova standard)
- Amy: 5, 6, 7, 8... Tall and tanned and young and lovely, the girl from Ipanema goes walking. And when she passes, each one she passes goes...
- Sheldon: (knock knock knock) Amy, (knock knock knock) Amy, (knock knock knock) Amy.
- Amy: ...aww
- (Amy stops playing and goes to answer her door for Sheldon.)
- Amy: You are aware that your ritualistic knocking behavior is symptomatic of obsessive compulsive disorder.
- Sheldon: Is not. Is not, Is not.
- Amy: Denial. Denial. Denial. Come in.
- Sheldon: (Trying to get Amy to help him with his disgust with Penny's chair) Name your price.
- Amy: Kiss me where I’ve never been kissed before.
- Sheldon: You mean like Salt Lake City?
- Amy: Never mind, I’ll talk to Penny.
- Sheldon: (To Leonard over his Long-Distance relationship with Priya): I miss the old days when your romantic partners could be returned to the video store.
- (Penny and Sheldon are reading magazines together.)
- Penny: We're like an old married couple.
- Sheldon: If we were an old married couple, the wife would be serving iced tea and snicker-doodles.
- Penny: I don't have any iced tea or snicker-doodles.
- Sheldon: A good wife would go to the store.
- Penny: I want a divorce.
- Sheldon: Good. On the way to the lawyer, go pick up some iced tea and snicker-doodles.
- (Both laugh over the joke)
- Amy: I just have one question about the chair.
- Penny: And what’s that?
- Amy: Aren't you worried about it being unhygienic?
- Penny: No, it’s completely fine. Hmm. I get it. Sheldon sent you. He put you up to this.
- Amy: No, he didn't.
- Penny: Really?
- Amy: Yes, he did. He absolutely did.
- Penny: My God, Amy, that’s really crappy of you.
- Amy: It is?
- Penny: Yeah! Letting Sheldon use you to manipulate me? I thought you were my friend.
- Amy: No, I am your friend. Please don’t be mad at me.
- Penny: I can’t even believe this. You know maybe you should just go.
- Amy: No, no, I’m sorry, I’m so sorry. I take it all back. Look. I’m, uh, I’m sitting in your chair. There’s nothing wrong with it. It’s a great chair. Please let me continue to be part of your world. Ow!
- Penny: What’s wrong?
- Amy: Something in the chair’s biting my tushy. It’s not important. Ow!
- Penny: Wait. Get up, get up! (She does. Something is moving in the seat cushion. They both run out of the apartment screaming.) Swear you won’t tell Sheldon what happened!
- Amy: (between screams): I swear! Can I tell my doctor? I’m probably gonna need shots!
- Penny: (also still screaming): Yeah, sure!
- Sheldon (at Penny’s door): (Knock, knock, knock) Penny. (Knock, knock, knock) Penny. (Knock, knock, knock) Penny.
- Penny: What’s up, buttercup?
- Sheldon: You have to get rid of the chair.
- Penny: Nope. (Closes door)
- Sheldon: (Knock, knock, knock) Penny. (Knock, knock, knock) Penny. (Knock, knock, knock) Penny.
- Penny: What’s the word, hummingbird?
- Sheldon: For your safety, please wait in my apartment as I call the authorities so they may remove the chair of death.
- Penny: No. (Closes door)
- Sheldon: (Knock, knock, knock) Penny. (Knock, knock, knock) Penny. (Knock, knock, knock) Penny.
- Penny: What’s the gist, physicist?
- Sheldon: Under my authority as a self-appointed member of the Centers for Disease Control street team, these premises are condemned. (Penny tries to close door) As a man with a keen sense of style, I must tell you, that chair does not work with the room. (Penny closes door). (Knock, knock, knock) Penny. (Knock, knock, knock) Penny. (Penny opens door and waves seat cushion at him. He screams and runs off. She closes door. He sneaks back) (Knock, knock, knock) Penny.
- ↑ The Big Bang Theory - The Infestation Hypothesis (TV Episode 2011) - IMDb
- ↑ The TV Critic's Review
- ↑ The Big Bang Theory: “The Skank Reflex Analysis”/“The Infestation Hypothesis” - TV Club - The A.V. Club
- ↑ Thursday Finals: 'Big Bang Theory,' 'The X Factor,' 'Parks & Recreation' and 'Whitney' Adjusted Up - Zap2it
- ↑ Top Programs – Total Canada (English) September 19 – September 25, 2011 - BBM Canada
- ↑ BARB via wikipedia
- ↑ ABC's Doc Martin beats Nine, TEN's best all week. - TV Tonight