Now, I've noticed many posts/comments/etc. on here stating that it is impossible to be a blood-sucking vampire. Quite frankly, I disagree.
To begin with, let's state the things that people generally assume about vampires.
- They suck blood
- They are fanged
- They are nocturnal
- They have pasty skin
- Their skin may be clammy or cold.
Now, let's describe each one. Humans can drink human blood if they so wish and manage to get the blood to drink. Most humans have "fangs," though we don't call them such. They are in fact our canine teeth. Humans also can be nocturnal if they choose. When one is nocturnal, they generally have pasty skin, unless they stop by the local tanning salon. The night generally tends to be colder than the day. Pale skin also absorbs less sunlight than skin with a tan. Put these two together, and a human's skin might be several degrees cooler than that of a human on a "normal" schedule.
Also notice that nowhere in there was DNA change mentioned.
Conclusion: vampires can exist, albeit humans with eclectic taste-buds and a unique schedule.