I'd think that Nick would be immune to the croatoan virus just because Lucifer's possessing him. Y'know, the way Jimmy is immune to stabs and gunshots because Castiel is in there. And sure, Nick is falling apart already because the body can't withstand Lucifer, so that would be Lucifer's primary concern with Nick, I think, not the virus. Lucifer has to move fast because Nick has an expiration date.

One thing I've always wondered: okay, if vesseling were passed on through bloodlines, does this mean Nick is related to the Winchesters somehow?