A scientific explanation is hard to find. A disease? There is some witch can be related to vampirism. But could vampirism really be a epidemic spread by blood exchange? This would explain a lot of things. Like the myth that says when you get bitten by a vampire you turn in one. But for that, he would need blood in is mouth which might be why not all bites are effective. And I don't think vampires have long pointy teeth and could ''suck'' the blood out of you.It would take a lot of time to suck a entire person's blood. Or even a small quantity.
Of all the disorders and diseases even loosely linked to vampirism, the most bizarre must be porphyria. It is a rare hereditary blood disease; its symptoms so closely match the myths associated with our modern conception of vampirism it's eerie. A victim of porphyria cannot produce heme, a major and vital component of red blood. Today, this disease is treatable with regular injections of heme into the body. However, as little as fifty years ago, this treatment was unavailable and the disease unknown.
In the past, a porphyria sufferer would show symptoms that include;
-Extreme sensitivity to sunlight
-Sores and scars that break open and will not heal properly
-Excessive hair growth
-Tightening of skin around lips and gums (which would make the incisors more prominent)
This disease would likely cause the victim to only go out at night, in order to avoid the painful rays of the sun. In addition, while garlic stimulates the production of heme in a healthy person, it would only cause the symptoms of porphyria to become more painfully severe. Porphyria was eventually discarded by scientists as a reasonable explanation of the vampire myth that has pervaded our history. Although vampire accounts of the past bear little resemblance to the dashing figure we romanticize today, these qualities may have contributed to our look at the vampire in film and fiction: pale skin, extended incisors, even the fear of the sun!