Bobby's about to make a smart comment about how most things that are really evil can't just be killed with brute strength, but he instead opens his mouth and then shuts it. The way he figured, having creatures that were pretty evil but could be killed just by stabbing them a whole lot was ... a good thing, considering some of the things Sam had told him could potentially come here from other peoples' worlds. Instead, he focuses on the part of Alistair's story that's actually relevant.

"So are we talking an injected venom from a bite, skin exposure to the stuff, or some sort of in-between?" This is important information for people who aren't immune. "And your ghouls - do they feed on humans, dead or alive? Ours do."

But then Alistair is asking him a question that he actually has to think about.

"Gotta be the Jefferson Starships." Never let Dean Winchester name anything. "Vampire-wraith-werewolf-shapeshifter hybrids. Nasty business, created when the mother of all monsters got it in her head to make the 'perfect creature'." Gross all around, basically.