it's a confirmed bug or feature (don't know wich) that Serana can infect you with Vamprism randomly if you have her as your follower, there was even a guy in the dawnguard forums a while back that had slept with her in his house's bed and when he woke up he got the message that he had contracted Vampirism
Possible to keep a couple just in case? what about that mystery machine everyone is on about?