Vampires are an ancient race. Their mythology dates back to the earliest recorded civilizations. In fact, most every culture has a myth of vampirism. In most cases, the vampire is the spirit of the deceased who torments those that knew him/her in life. The torment often takes the form of blood drinking (sometimes of the menses), stealing the unborn children of pregnant women, or draining the life force of a person.
The cause of vampirism varies from culture to culture. Simply not completing the proper burial rights can cause the dead to rise and seek vengeance, as with the Indian bhuta. Revenge is at the heart of many a vampire myth. In Arabian lore, an afrit is the vampiric spirit of a murder victim who seeks to avenge his/her death. Likewise, the Gypsy mullo will rise to avenge its death, often by sexual assault.
Not all vampires are made, though. There are demons of supernatural descent. Among the earliest of recorded vampires is the Greek lamia. Lamia was the lover of Zeus. When Hera, the wife of Zeus, discovered his infidelity, she killed Lamia's offspring. In her anger, Lamia swore to kill all children. The demon lamiae suck the life out of newborns. Even the Hebrews believed that Lilith, Adam's first wife (and current symbol of feminism in music), was a lamia.
Obviously, the true vampires are fearsome demons from the Dark Side of our psyche. Vampire myths were used to explain many natural and human-made phenomena. Fear of sickness, death, darkness, and the Unknown is the blood that gives vampires life.