This just popped into my head while reading other posts.
Harry baseballed Sith into the lake but he's THE Cat Sith. No way that killed him.
So, unless Harry will or has told Mab that Sith was taken, he can still infect other Sidhe..
And, I mean, harry knows Sith's power; he has to know Sith ain't dead. I'm hoping he told Mab between books, and gets cured, because he was a nice foil for Harry.