Vampirism
Generally believed to be the result of an elder curse befalling an unfortunate bloodline, vampirism is an extraordinarily rare and dangerous condition that has been known to lay dormant for many generations before arising in an individual.
Causes
Vampirism begins in death. The bearer of a vampiric curse who dies under particular conditions can rise as a vampire. The conditions are poorly understood, and often related to the particular lineage of the elder curse the vampire's family is under. Regardless, an unfortunate carrier can rise as a vampire after death, and it has been noted that those who have died prematurely are more likely to rise as vampires. Rising from the grave, the vampire then realizes that they have been brought back to the world, but require the blood of the living to sustain themselves.
Another way to become a vampire is to be purposefully turned by another vampire, though the lineal curse is not transmitted this way.
Symptoms
The vampire, rising from death, appears as one of the just deceased. Generally able to pass for living, the vampire nevertheless finds themselves with many new restrictions and demands on their undead body. Though supernaturally strong and agile, sleepless, and possessing the power of hypnotism, the vampire is also nourished by the blood of the living, and must thus prey on their former society to avoid wasting away.
In an unexplained peculiarity of the vampiric curse, the vampire is harmed by running water and sunlight, and cannot enter the dwelling of another without direct permission.
Treatment
There is no known cure for vampirism, save a stake through the heart.
Cultural Reception
Vampires are rare enough to be generally considered a fell myth, though those in the know understand that this myth is true. The carriers of the elder curse are understood to descend from someone who consorted with Borderworld demons, and received the curse of undeath in return.
I really like the idea that vampirism is mostly caused by a generational curse. That's a nice twist.