Here is truth - read and understand. BTW, this is my own answer, endlessly recycled, and I will continue to use it, despite false allegations of plagiarism.
1. Vampires, defined as a humanoid being that MUST consume blood or energy to survive, do not exist. Cut and paste time, as it is too much work to type this out over and over and I "recycle" my own answers instead of retyping them so here goes. A brief discussion of the human digestive system and then the probable vampire population given an exponential growth rate should explain why vampires are not possible.
2. These humans that affect the whole "vampiric lifestyle" are NOT vampires. They are simply humans playing their own little game, in their own little fantasy world, usually pandering to their own little sexual fetish, which may or may not actually be sexual. I too, play my own little game, in the SCA, but mine is a game where the deeds that I do are determined by the strength of my arm and sword - I am a warrior, with just as much skill and ability as any warrior of ancient times. The difference is that I am claiming to be something physically possible: a warrior, and I prove it everytime I strap on my armor and walk onto a SCA battlefield. The so-called "vampires" are claiming to be something physically impossible: a walking corpse, and all they prove is that black Victorian clothing, a pair of false fangs, and a little makeup make for a good Halloween costume - it does not make you a vampire.
3.The humans who profess to be vampires are victims of an all-encompassing self induced delusion. They are as human as you or I, regardless of their claims, and if they ingest HIV tainted blood they can most certainly contract the disease, esp. if they have any cuts, sores, or lesions in and or around their mouth. It is a very dangerous delusion to be laboring under. Note that there is absolutely no scientific or medical proof that these people derive any benefit at all from the ingestion of blood, and even worse are the so-called "psychic" vampires, because their delusion is one that they cannot substantiate with any concrete evidence at all.
4. There is no "vampire" gene. People are not "born" as vampires. When a woman goes to the hospital for prenatal care there are many tests done on mother and child, even while still in the womb, to check for many things, including genetic anomalies that result in deformities and birth defects. If such a gene existed, in today's world with today's technology it would have been found - we have already completely sequenced the human genome. It would also have to follow Mendel's law of dominant/recessive gene theory. Again, the odds on that many "vampires" all escaping the notice of the medical/scientific community are so low as to be almost nonexistent.
5. There is no "vampire virus" - as I have already pointed out, HIV is a virus, and look at how fast it has spread - virtually everyone knows someone with the affliction. According to the "vampire websites" there are "thousands" of vampires running around. If that was so then at least one of them has ended up in a hospital for bloodwork when they became pregnant, had a bloodborne infection, was injured in a car wreck, etc, etc, ad nauseum. The anomaly would have been detected and medical science would have isolated it, studied it, applied for research grants on it, published papers on it, and turned it into the talk of the medical and scientific community, as well as making its "discoverers" celebrities and rich beyond their dreams. A virus cannot alter your DNA in such a radical fashion without killing you.
I am a Nurse, real education, real experience. My answer is completely based on my knowledge and training, and is not copied from any outside source. Pay real close attention to # 4.