The Alien Parasite Hypothesis (2010)

Amy becomes attracted to Penny's ex Zack, whom she meets in a bar on a night out with Penny and Bernadette. She later discusses her symptoms (elevated heart rate, clammy palms, dry mouth, and an involuntary "hoo") with Sheldon and comes to the reluctant conclusion that she is horny. It is also pointed out that Sheldon might be jealous. Penny tells Sheldon there is "something" he could do in this matter, however, as usual, he misunderstands her point and makes a number of phone calls until he finds the right Zack to set up a date for Amy. This proves fruitless as Zack's limited intelligence puts her off. After leaving the bar, Amy grasps Sheldon's hand. He asks her why she is doing this, to which Amy replies that she is conducting an experiment. After a short interval, she lets go and says, "Nope. Nothing. Never mind.", indicating that she is not attracted by Sheldon's appearance, however chose Sheldon's intellect over Zack's looks.

After an employee in the bio lab is bitten by a radioactive rat, Howard and Raj quarrel over who would be the better superhero (with Raj expressing his desire to be "Rat-Man"). Intending to use Raj's fear of spiders, Howard proposes a contest where the one who can keep his hand inside a container with a giant spider the longest, wins. This, however, backfires as the spider crawls up Howards arm, freaking him out. In the end, they decide to settle their dispute with a wrestling match between them, with Leonard being the referee. Neither of them gets touched as they dance in circles and insult each other for hours.

Recurring characters: Brian Thomas Smith as Zack Johnson.

Title reference: One of Sheldon's theories about why Amy had her symptoms, which he thinks is more likely than her being sexually aroused.