Every time you get hit by a vampire you have a chance of becoming one yourself.

Although it can be cured with a standard disease potion, if you leave it for 3 days it will become a 'permanent' effect (you can change back but it takes alot of work, so dont make the decision lightly).

Once you have passed the 3 day period you go through four stages of 'the change', each increasing both your power and the hostility of the NPCs towards you.

You should also be aware that while you will become more powerful in many aspects the longer you go without blood you will also become weaker in many other ways, which eventually lead to the point where you can only drink blood from sleeping NPCs, which can be a real headache to find.