The Alien Parasite Hypothesis

The Alien Parasite Hypothesis is the tenth episode of the fourth season of the CBS sitcom The Big Bang Theory. It aired on December 9th, 2010.

Plot Summary
As Amy starts to have her womanly urges, Sheldon must question what his relationship with her means.

Expanded Plot
Amy begins to have feelings for Penny's ex-boyfriend Zack, whom she meets in a bar while on a night out with Penny and Bernadette. Amy and Sheldon later discuss her symptoms (elevated heart rate, clammy palms, dry mouth and an involuntary "hoo!") and come to the reluctant conclusion that she is horny. Penny tells Sheldon there is something he could do about that, so he, missing the point, tries to set Amy up a date with Zack. This proves fruitless as Amy finds Zack as dumb as Penny does. As Sheldon and Amy are leaving the bar, she grasps his hand. He asks her why she is doing this; she replies that it is an experiment. After a short interval, she lets go and says, "No, nothing. Never mind."

Meanwhile, Howard and Raj quarrel over who is the better superhero and sidekick between them. They get Leonard to referee a wrestling match between them, but neither guy gets touched as they dance in circles for hours.

Quotes
Leonard: We get it! 73 is the Chuck Norris of numbers.

Sheldon: Chuck Norris wishes.

Howard: Let me ask you a question. What are you most afraid of?

Raj: I don't know. Nuclear war. Accidentally being buried alive. Any of those movies where you get a phone call that says you're going to die, and then you do.

Amy: HOO! (often repeated)

Trivia

 * The title of this episode refers to one of Sheldon's theories about why Amy had her symptoms.
 * Howard states, "I’m legally obligated to inform you that I took a karate lesson when I was 11. I’d be a regular ninja by now if my mom could’ve arranged a carpool." In "The Jiminy Conjecture," he similarly revealed, "Don’t push me, Sheldon. I may be small, but I took kung fu when I was 13, and I remember a good deal of it."
 * Rajesh and Howard argue superhero names Rat-Man, Mouse-Boy and Kid Vermin for themselves if one was bitten by a radioactive rat in a lab. Akin to this, in "The Hot Troll Deviation," Rajesh said if Sheldon's superpower were arrogance, his name would be Captain Arrogant, while Sheldon contended his name would be Dr. Arroganto.
 * This episode marks the first contest in a gym, with the second occurring in "The Rothman Disintegration."
 * Sheldon shows his favourite character Spock's salute (from Star Trek).
 * In "The Lunar Excitation," Amy said, "I don’t object to the concept of a deity, but I’m baffled by the notion of one that takes attendance." Amy suggests religion, in the episode of this article, when Sheldon says, "You know, in difficult moments like this, I often turn to a force greater than myself." This reaffirms she does not object to religious beliefs.
 * While speaking to Zack, Sheldon makes a reference to "The Justice League Recombination": "Good grief. It’s like trying to talk to a dolphin."
 * Sheldon and Amy's discussion of her physiological reactions (HOO!) to Zach is an homage to Abbott and Costello's famous "Who's on First?" routine.
 * This is the first time that, unlike Sheldon, Amy shows that she has sexual arousal.
 * "Lupus" on diagnosis board may refer to the TV series House, M.D.
 * Sheldon's favorite number 73 is coincidentally the episode number of this episode