Mostly from Pravda.ru:
There is a disease called Porphyria cutanea tarda that presents clinically as a pathological sensitivity of skin exposed to light causing scarring, hair growth and disfiguration. Additionally, it was believed that the patients’ missing heme could be absorbed through the stomach, correlating with the legends’ hematophagy.

This is a rare disease – only 1 in 20000 people suffers from it. The body doesn’t produce red corpuscles and thus a person’s blood lacks oxygen and iron and this leads to hemoglobin breakdown under the sunshine. Soon blisters and ulcers pop up; a person starts to have sun energy and can even die. This disease can also cause nose, ears and cartilages deformation. His fingers start to convolve; the skin around mouth gets dry and reveals gums, which turn yellowish because of porphyrine deposition on the teeth. Garlic that stimulates red corpuscles emission in the body of healthy person causes the exacerbation of symptoms among the ill people. This goes hand in hand with harsh pain, so these people also often suffer from mental disability.

If you sum up all the symptoms of this disease, you get the exact same picture of a vampire shown on TV. In France only in the 17th century 30 thousand people were declared werewolves according to the signs described by Illis. All of them were hanged. The Czech archeologists found the burying that dates back to the 11th century. There were 13 people lying with tied hands, chopped off heads and stakes in the chest.

As you can see, vampires of a sort do exist.

In England a 17-year teenager killed his neighbor, cut her heart out heart and sucked the blood to gain immortality. –I doubt that his dementia? has anything to do with the disease. However, when you combine the deed of a single individual with the physical signs caused by extreme cases of the disease belief in monsters becomes a given.

Even with the diagnostic science available to modern medicine I would wager that quite a number of otherwise intelligent individuals still believe in vampyres and werewolves.

Even well educated individuals can be found who still believe in curses, witchcraft, and an unbelievable amount of “unexplained phenomena.”

The best example of people’s insistence in their belief in the supernatural is best demonstrated by a psychological test in which a group of individuals is told that a “psychic” has done a reading on each of them and they are each given the results. These “results” are all identical.
When the volunteers are shown this, a certain percentage will get angry and insist that in spite of this basic demonstration, that not only must such things exist but that this somehow proves it.

Welcome to the human race.