"Invasion" is the fifth episode of the sixth season of Grey's Anatomy and the 107th produced episode of the series. It is also the 140th produced episode of the Grey's Anatomy franchise. It was broadcast on October 15, 2009.
The residents of Seattle Grace and Mercy West vie for surgeries and their careers as day one of the hospital merger begins. Alex finds themselves in direct competition with the Mercy West residents while the pressure of fighting for her career takes it's toll on Cristina. Izzie faces the brunt of her new surgical competition. Meahwhile, Callie looks to Arizona for support when her father returns to Seattle and employs a priest to help convince her to date men.
- "When you get sick, it starts out with a single bacteria. One lone, nasty bacteria. Pretty soon the intruder duplicates, becomes two. Then those two become four and those four become eight. Then, before your body knows it, it's under attack. It's an invasion, the question for a doctor is once the invaders have landed, once they've taken over your body, how the hell do you get rid of them."
- "What do you do when the infection hits you? When it takes over? Do you do what your supposed to and take your medicine, or do you learn to live with the thing in hopes that someday it goes away? Or do you just give up entirely, and let it kill you?"
- "Clap (See The Stars)" - The Myrmidons
- "I Want You Now" - Traildriver
- "Your Ghost" - Greg Laswell
This episodes' title references the song Invasion, originally sung by Iron Maiden