This problem occurred to me yesterday (2/7/2012) and I wonder if anyone else has this problem. Ever since the Dawnguard update things have been a little strange. Butt that is not the problem. The problem is I randomly become a vampire. I receive no warning with it saying "You have contracted -Whatever the name is-" and I have travelled back before the event, used cure potions, cured at shrines and yet I still get vampirism. And I'm skilled at archery so I never take a hit from a vampire. But the most confusing thing is that I'm a werewolf, so I have 100% disease immunity, and I wear a 100% disease immunity necklace ALL the time.
If you are having this problem please post your problem, or post if you know the cause.
-Top_agent009 
P.S: My follower is Serana