I think that the parasite is Lash. The timing is too convenient. We know that Mab did some pretty impressive healing on Harry. She left the scars, but those are just cosmetic anyway. There's no reason to believe that she didn't restore the burned out parts of his brain in the process.

That's why I also think that the shadowy whisper wasn't Lash. It showed up pre-sexual healing. As to who it was: my money is on Anduriel or ol' Lu himself. I lean toward Anduriel, because the Nickelheads have a good reason to try to influence Harry. Harry was able to do two things which nobody has ever done before: shake off the coin's influence without renouncing his magical power in the process, and come thisclose to offing Nicodemus. They would either want him on their side or dead. If they get him especially desperate, he'd either take up the coin or do something desperate against the Red Court, which would kill him.

Oh well. None of it matters. The world is going to end in 2012, and now we know why: a certain coroner decided to introduce a certain Spirit of Intellect to a little thing called the internet. BOB IS ONLINE! WE'RE ALL GONNA DIE!