If someone was extremely allergic to sunlight, silver and onions, unbelievably obsessive-compulsive, and developed some kind of clinical vampirism, would that person be considered a vampire? If the answer to that question is yes, then at least one vampire existed at some point over the course of human history. Also, I will dedicate my life to finding a person allergic to all those things and brainwashing them into thinking they're vampires. If not, I will dedicate my life to the creation of the actual vampires (the horrifying Slavic version) by the means of nanobots and DNA engineering and let them ruin the world. All for the lulz.