My conjurer sided with the Dawnguard against the Volkihar and slaughtered them all. He carefully considered the morality of it, and finally decided to reanimate Modhna as a thrall. He's not really into necromancy, but had stumbled across the Dead Thrall spell as loot. He figured reanimating an evil vampire to fight on the side of good was not a moral issue. But there was a fly in the ointment...

I recently noticed that he had turned into a vampire, despite not fighting any in the near past. What the hell? Then I realized...he must've gotten caught in one of Modhna's death rays during combat and had been turned. I just didn't notice the Sanguinaris Vampiris effect.

That's the kind of thing I love about this game. Meddling in the dark arts *does* have consequences. If I stumble across Falion during my adventures I'll undo the damage, but I'll roleplay it without metagaming.

Best one for me is Mogrul at Raven Rock. Dual-cast fury on him and let the town savor the beat down on him... pay back is a -----

Wait, so you can catch vampirism from thralled vampires?

So it seems. I'm about 90% sure I haven't fought any vampires lately, yet here I find myself turned into one. I may experiment with this later to verify, but it seems the most likely explanation.

This too happened to me fairly recently. Perhaps 1.9 had something to do with that?