Can you say Invasion of the Body Snatchers anyone?
"THERE HERE! THERE HERE ALREADY! WE'RE ALL IN DANGER! THEY'RE EVERYWHERE! YOU'RE NEXT! YOU'RE NEXT!"
As for who the "pod people" a.k.a Skrulls are:
*Charles Xavier is a good guess. It gives them all kinds of access in terms of monitoring the mutant community, especially if they managed to replicate Xavier's power.
*Maria Hill I think is more of a useful idiot rather than a Skrull, although certainly someone on SHIELD is one.
*Spider-Woman could be a good guess too, as that New Avengers #32 preview suggests that she has had access to both SHIELD and HYDRA, and my guess would be that she was replaced sometime during the Savage Land story.
*Luke Cage and Jessica Jones are not Skrulls, but the baby sure is.
*I don't think Mary Jane would be a Skrull, although it would appear to be a simple "solution" to the Spider marriage, because: 1. It's a retread of how Marvel retconned Johnny Storm's marriage to Alicia, and 2. If she's been a Skrull all along, Spidey still married her and if she's killed it makes him a widower which "ages" him and that's something Joe Q has said over and over he doesn't want, or if she's been a Skrull since MJ disappeared in ASM #13 vol. 2, it still means Spidey was/is married to her and if she is in fact dead Spidey's still a widower. No, my guess is "One More Day," which is supposed to resolve the marriage question, will use some other ridiculous way to do this, especially since it's supposed to tie into things JMS did during his entire run. Then again, I certainly wouldn't put this past Marvel.
*Clint I don't think is a Skrull either. He's just the HOM version of Clint misplaced in the current 616 reality.
*Norman Osborn as a Skrull I could easily see Marvel doing, especially to satisfy long-time complaints that he should never have been brought back to become a Lex Luthor rip-off.
*Tigra as a Skrull would make sense, especially in light of her role in Civil War and her seducing Hank Pym.
*Miriam Sharpe as a Skrull would also be something I'd see Marvel doing, in that while they may not have started Civil War, they certainly nudged things along. (I'd suggest Hulkling falls into the same category.)
*Interesting how some are suggesting Storm is a skrull, but what about Black Panther?
*I'm guessing Somebody inside the Red Skull's organization is a skrull and is manipulating him, especially it light of killing Captain America, who definately was not a Skrull.
*I'm probably a Skrull so you can't trust anything I say anyway.

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