I am an entire day behind the television watching, so for the sake of time, reviews for Thursday and Wednesday shows will be very short (shorter than they the present short reviews). I was overwhelmed this week with other stuff, but I'll definitely be churning out reviews next week.
What happens in "The Alien Parasite Hypothesis"? To be honest, nothing--literally. Amy Farrah Fowler finds herself attracted to one of Penny's ex-boyfriends, making involuntary noises when looking at him. She talks to Sheldon about it and they try to deduce her urges. Up to that point, everything is fine. When Sheldon takes her to get laid, however, Amy turns back, deciding that the guy is too dumb, and leaves. She holds hands with Sheldon, doesn't react, and lets go. Basically, the episode was about Amy miraculously attracted to one person. There are no bigger implications.
As for Howard and Raj, what the hell? How about letting them run around in circles singing? That's almost equivalent to their ridiculous superhero-sidekick competition.