What is Wrong with Izzie Stevens?

We're all wondering it, and E! Online has put together a list of theories and possible (using the term loosely, of course) explanations for Season Five's most intriguing plot line: Izzie's "visions" (and more) of Denny Duquette, her fiance who bought the farm in Season Two.

Which do you think is best? Do you have your own? Comment below ...

Theory #1: Izzie Has a Brain Tumor

Evidence: Symptoms of a brain tumor include: inability to concentrate (check), gradual changes in intellectual or emotional capacity (check), lethargy (check), changes in behavior or personality (check), and/or hallucinations (check).

Verdict: Debunked... or so we think. Shonda Rhimes told TV Guide's Michael Ausiello, "I think the love triangle with Denny, Izzie and Alex is among the most interesting we've ever done. Watching the chemistry between Jeffrey and Katherine again has been really touching. I can't wait for our viewers to see where we're taking it. But what it won't involve is Izzie having a brain tumor." Okay ... unless Shonda is just trying to throw us all off.

Theory #2: Izzie Is Losing Her Mind

Evidence: She is having hallucinations of her dead fiancé. Active, ongoing hallucinations where they have tons of sweaty sex and he incessantly follows her around. If it looks crazy, sounds crazy and smells crazy, it probably is crazy. Even the Seattle Grace nurses have blogged that she's out of her right mind: "Izzie Stevens has lost it. I mean it. She's gone... I hope things get better for her because it looks like Izzie can't take one more bad thing happening ..." Could the nurses know more than we think they know?

Verdict: Izzie could very well be on the verge of a nervous breakdown, which could definitely have implications for her relationship with Alex. His mom was nuts, and Ava (Elizabeth Reaser) had a devastating breakdown of her own last year - a situation, it's worth noting, that Izzie helped Alex endure.

All this does beg the question, though: Why is Alex so crazy-making? Is he just too hot? Does his pure sexual energy fry the brain?

Theory #3: Izzie Is Dead, Too

Evidence: We all saw The Sixth Sense. Everything is going right for Izzie, other than seeing her dead ex-fiancé. She has roommates who love her, boy toy Alex (Justin Chambers), who finally wants to be with her, and a job she loves, despite the fact she stole a heart... Is it possible that it's all just a happy limbo where Izzie has dwelt since she killed herself after Denny's first demise?

Verdict: As far as the rest of the characters are concerned, Izzie is alive and well, but who knows? And Mer has communed with dead Denny, dead Dylan and dead mom Ellis.

Theory #4: Izzie Has Post-traumatic Stress Disorder

Evidence: The girl is way too busy fighting for surgeries to even take a moment to deal with the loss of her fiancé. This has happened once before, when Izzie was too active being an underwear model to face the emotional reality of putting her daughter up for adoption.

Verdict: Izzie's own best theory on what's happening to her is a delayed grief experience, and who's to say the doctor can't diagnose herself...

Theory #5: Izzie Has an Overactive Imagination

Evidence: Plain and simple, sex with a dead man is not possible, even if it is... fulfilling. Co-executive producer Joan Rater said on the Grey's Anatomy Writers' Blog: "Izzie knows this isn't real, she knows that, but it feels real and she just ... closes her eyes and, you heard the sounds coming from the room, as Meredith said, it sounded kinda porny, so whatever's going on with the dead guy, the sex is good, at least ..."

Verdict: If this is the love story the writers were hinting at for Katherine Heigl ... she's essentially having a love story with herself, and that's not healthy.