It's been known to happen... but not to anyone I was friends with. [Faith doesn't have any examples of friend turned vampire, but she knew of instances where it had.]

We were able to beat them by the skin of our teeth, is how. With all the Slayers, this magical scythe, a witch, this amulet that a vampire turned good wore that... well, he wound up incinerating himself to save us all...

[Her and Spike might not have gotten off on the best foot, but he was alright. All their animosity had been because of Buffy.]

Well, hopefully we get some bigger brainiacs in this place than me. And they can help us figure it all out.