The following information describes afflictions and diseases that could cause one to be mistaken for a vampire, and scientific “reasoning” as to why one would think themselves to be a vampire.
A blood disease in which the red-cell count of the afflicted is abnormally low. Red blood cells carry oxygen through the body. Symptoms may include: a pale complexion, fatigue, fainting, shortness of breath and digestive disorders. Anemia can be caused by disease, heredity or severe blood loss. Over the years, people suffering these symptoms were often considered to be victims of vampiric attack. The afflicted person’s pale complexion and difficulty eating food often caused suspicion that one was becoming a vampire. [More about anemia on WebMD]
A nervous system disorder that causes loss of voluntary motion, muscle rigidity and decreased sensitivity to pain and heat. A person suffering from Catalepsy cannot move, but is still able to see and hear. Their breathing, pulse, and bodily functions are slowed to a point that they can be mistaken for dead. Before the 20th century, there were inadequate methods for determining whether or not one was actually dead, and embalming was not yet in practice, which may have caused some sufferers of this condition to be buried while still alive. Upon waking from the cataleptic state, the person may have tried to dig themselves free of the grave, thus explaining the stories of the “undead” rising from the grave. [More about catalepsy on MedTerms]
A rare hereditary blood disease in which the afflicted is unable to produce heme, a major component of red blood. Symptoms include: photosensitivity, paling of the skin, , sores and scars that will not heal, tightening of the skin around the lips and gums (thus making the teeth appear more prominent). Until about fifty years ago, there was no known cure for the disease, which can now me alleviated with injections of heme into the body. The disease would cause one to likely only venture out in darkness to avoid the painful rays of the sun. Also, while garlic is beneficial to a healthy person, it would only exacerbate the symptoms of someone suffering from Porphyria. In times past, it was believed that Porphyria could be cured through blood transfusions, which may have added to the myths of people drinking blood, most likely in an attempt to alleviate their symptoms, rather than as a vampiric act. [More about porphyria on WebMD]
An extremely rare genetic condition in which the afflicted has such severe photosensitivity that exposure to UV rays, particularly from the sun can cause irreparable skin damage and even death. Symptoms include: irritation and blistering after minimal sun exposure, premature aging of skin, eyes, mouth and tongue, blindness, developmental disabilities, high frequency hearing loss and deafness. Sufferers of this condition, like those with Porphyria, would need to be completely sheltered from all sunlight, most likely venturing out only in total darkness. The severe reaction of pain and burning of the skin suffered by victims of XP could, before times of proper diagnosis, have led one to believe that the person was a vampire, since it was commonly thought that vampires were killed by sunlight. [More about XP on XPS.org]
There are those who claim to be able to leave their physical body, and move about on the astral plain. If someone was known to be capable of this, it could have led to them being thought of as a witch or vampire. If one were to be visited by the astral body of a person, they may have thought they were being contacted by a vampiric spirit.
Ignorance, the Church, and Lack of Medical Technology:
Before the advent of modern technology, many things would lead one to believe that someone was a vampire. In addition to the previously mentioned symptoms, if one were to be exhumed, and their hair or nails had grown, their skin appeared flushed, their skin was warm, or they had blood at the mouth, they may have been considered to be a vampire. Today we know these to be normal occurrences after death and during decay. It was often considered that a male who had been exhumed and possessed an erection was a vampire. This was actually caused by the expanding gassed in the penis and scrotum that naturally occur after death. This may have led somewhat to the eroticism of vampires, as one would have believed the man to be sexually virile even after death.
Blood on the hands, or a ghastly expression of the face of the exhumed body were also considered signs that they were a vampire. These things were usually caused, in reality, by premature burial due to the inability to properly diagnose a severe health problem. If a body was exhumed and staked (the most popular way of “killing” a vampire, next to beheading), and it groaned or “breathed” it was considered proof that it had been a vampire. We now know that it is not unusual for a body to expel gasses, especially if punctured or impacted, and this could lead to the deceased making “unnatural” sounds. Widespread diseases, such as Bubonic Plague, were often attributed to vampires, and the first person to be afflicted was typically considered to be the vampire. The church had a great deal to do with this. Since there was not effective medical science to explain the plagues, the church looked for supernatural forces to blame.