Scientifically, there is a virus called endogenous retrovirus that makes you depressed and have a craving for blood. Back around the time of the black plague, people with this virus were considered real vampires and that's how the myths started. http://www.vampirewebsite.net/ for more info
I would like to point out that the only connection between ERVs and vampirism are sites like the one you provided, which do not have credible backing. From a scientific standpoint, to briefly explain HERVS (human endogenous retrovirus): " HERVs represent the remnants of ancestral retroviral infections that became fixed in the germline DNA..they have acquired many mutations over the course of evolutionary time so that, with a few exceptions, they are now defective and incapable of producing protein..HERVs have frequently been proposed as etiological cofactors in chronic diseases such as cancer, autoimmunity and neurological disease. Unfortunately, despite intense effort from many groups, there remains little direct evidence to support these claims"
In short, HERVs are essentially nothing more than "junk DNA" created from previous flaws in a genome and passed on to the next generation. This junk DNA has not been scientifcally proven to be related to any specific disease(s). They're still unsure about what its actual functioning role is. It does not relate to anything having to do with vampires. I'm not saying vampires are real, they aren't . But you're mistaken in your facts about this topic.
Here's a scientific article you can read to understand what HERVs are better:
true, also there's a mental condition, mental illness that is caused usually by cabin fever, sometimes it's a genetic brain illness disorder that comes naturally at birth, it causes the person if the person is stuck with people for too long then they go mad and the want to either eat your flesh or drink your blood, the reason why the cabin fever gives a person a thirst for blood or flesh is because they want to be closer to you, it's in their minds that they can't get enough of you, and the believe they need you inside them in order to be free from the person, some disorders cause someone to think the same way but it's not caused by cabin fever, it's just a mental illness that causes them to think they can't get enough of you and they need a piece of you inside them, they think it's romantic sharing blood or drinking blood, also the way the myths got started was because some people had a allergic reaction to the sun or certain metals that caused people to think they were vampires or werewolves, and some people were albinos and they had red eyes or pale skin and they also had a sensitivity to the sun, you can believe whether or not they are real, I think the vampire was a symbol of evil inside people, and they were a symbol of all the viruses in the world would infect people with a disease and people would die, it was a symbol of being almost dead but still alive by having a disease that would slowly kill the person, it was just a symbol of the suffering people had with the disease while they were still alive, of course we don't know whether or not vampires exist, it will always be a mystery of the unknown things in this universe, will I tell you whether or not I believe in vampires, of course not, can any of you tell us whether vampires exist or not, probably not, it should remain a secret, and if someone is a real vampire they should keep it a secret, they should keep it a hidden knowledge, because the way people react to the things that are mysterious or unknown they react in fear, violence, discrimination, persecution and death. If a vampire was real you bet your butt people will try to kill it, torture it, or torment it, no matter if they are evil or not, you bet on your butt they would destroy the being. I think it's best to keep it a mystery, and it's funner that way because people can fantasize about it however they want in their minds, they can think of them as good beings, maybe they will make more stories like twilight, maybe they will think of vampires like edward the lover, or the mysterious alluring tall dark and handsome figure that has a dark side to him or her that makes them irresistible, but if we knew if they were real our imaginations would be broken, and people wouldn't like them anymore, or their thoughts about them would be turned around and they would hate them instead of loving them, and I think the vampire is a symbol of our fear of the dark or darkness in this world, even literally people are afraid of the darkness, kind of like how a child is afraid of a dark room and thinks he or she sees monsters, which in reality it's their imaginations or fear of what they cannot see or touch, or the fear of the unknown, I think vampires are the shadow of our minds, the darkest places in our minds that have a never ending thirst for power, evil, destruction, hate, fear, and strength, I also think the vampire is our thirst for survival and our subconsciousness, the evil and hatred that's fueled by anger, fury, hate. And the vampire is a symbol of our darker selves, it's also a symbol of our curse of being tormented by evil people, it's a symbol of our curse of living each day in sadness, depression and sorrow, it's a symbol of always repeating the day the same way and symbolically dieing and living at the same time, meaning we die in sadness and yet still live to be tormented by ourselves, our guilt, our sins, our troubles, it's almost like living while dieing, it's kind of like being undead. And a creature like a werewolf is the symbol of our rage, fury, survival, and primal instincts, and the curse of the moon is the symbol of madness or love, the curse of the moon is a symbol of light with darkness, it's the symbol of being a lunatic, a symbol of being a zombie is being controlled by other people, the modern system of life, how the government controls you to pay taxes and the symbol of working your whole life to survive on money for food and water, a house, and material needs, anyways the point is is that all mythcal creatures have a symbol behind them which live inside us all, for one last example the unicorn is a symbol of imagination, life, purity, love, and kindness. Anyways whether you believe in vampires or not it's your choice, but even if they aren't real, vampires are just the darkness or romantic side is within us all.