Vampires do not exist, and this can be proven with the application of logic, critical thinking skills, and verified scientific and medical data:
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. The human body is not designed to process large amounts of blood for nutrition. There is not enough protein, carbohydrates, and fats present in blood to maintain a complex creature such as Homo Sapiens or any theorized offshoot mutations. When a human ingests food it is first broken up into a bolus by chewing, then churned up in the stomach with digestive juices to form a mass called chyme. It then passes through the pylorus into the duodenum, part of the small intestine where it mixes with bile salts and secretions from the pancreas and liver which continue breaking it down on a molecular basis, mostly affecting fats at this point. The broken down nutrients pass through the wall of the intestines and into the bloodstream where they are carried to each cell or stored for later use. Indigestible bulk continues through the intestines, turning a dark brown from the bile. Water is absorbed from this mass in the large intestine depending on the needs of the body - a well-hydrated person will usually have a softer stool than a dehydrated person will. Water also enters the bloodstream, and this is what helps to maintain blood pressure. The pressure tends to balance itself in a healthy person because the bloodstream goes through a formation in the kidney called the Loop of Henle, where the narrowing blood vessel forces excess water and cellular waste such as urea out through the cellular wall into the kidneys, where it is excreted through the ureters into the bladder, and then out of the body via the urethral passageway.
3. IMPORTANT - A person physically unable to process his own food for nutrition therefore also could not process blood - it's the same process. Ingested blood does not transmit directly to the veins anyway - it would be chemically broken down by the digestive system.
4. Theoretical ingestion of blood to supply these nutrients would therefore have to occur at least once a day, and would require the ingestion of the entire blood supply which could not happen as the stomach is far too small to hold that much liquid volume. Hold up your clenched fist - under normal conditions your stomach is about that size. Furthermore, such a mass would be difficult to pass thru the intestines as it has no fibrous bulk, would create an intestinal impaction, causing massive vomiting from the large concentration of iron present, and any "real" vampire would have to eventually expel the waste, which would come out as a black, tarry, smelly goo, just as stool does when blood is present from a upper GI bleed.
5. 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.
6. Even if a vampire feeds once a week, and his victim also becomes a vampire, that is exponential growth, with four iterations a month. First iteration: One makes one, total two. Second iteration: Two make two, total four. Third iteration: Four make four, total eight. Fourth iteration: Eight make eight, total sixteen. 16 vampires at the end of one month, 256 at the end of the second month, 4096 by the end of the third month, 65,536 by the end of the fourth month, 1,048,476 at the end of the fifth, and 33,572,832 vampires at the end of half a year! By way of comparison, there are currently approximately 33 million people who have HIV/AIDS and the disease is a world-wide epidemic. I see people every day in the hospital with AIDS, but never has there been one documented case of a vampire attack. Do the math - vampires are a mathematical impossibility.
As for the idea that vampires existed "a long time ago" consider the estimated global population 5,000 years ago - using the above mathematical rationale, a single vampire could have converted every human on the globe in less than six months. This falls therefore, under the logic of Occam's Razor - which states that when you have removed every impossible answer, whatever remains, no matter how improbable, must be the truth. Since there is no "vampiric plague" swarming the earth, the logical deduction is that they don't exist.
7. Point of clarification about "vampire" bats: vampire is simply the name we have given them because they do drink blood, same as a flea, mosquito, leech, or spider. Are these creatures vampires? No. They are living creatures, not legendary monsters. They can subsist on blood because of their smaller size and proportionately larger stomach volume. Drinking blood does not make you a vampire anymore than eating raw meat makes you a werewolf, although it might make you a mosquito.
8.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.
9. 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. The idea that there is a global "vampire community" engaging in controlled breeding to keep the "bloodline pure" is delusional in the extreme.
10. 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.