Porphyria - Porphyria is actually a set of rare diseases; and the rarest forms of the disease cause sensitivity to light, facial deformity, and increased growth of hair on the face. There are also prominent mental symptoms, such as hallucinations, anxiety, mania, and others. It would not be hard for one suffering from the disease in the medieval era to become convinced they were a monster of folklore. It would be even easier for others to believe it, because of the strict church run government, and folklore saturated life.
Celiac Syndrome - Celiac syndrome is an affliction which is described sometimes as a mental disorder, and other times as a physical illness to do with the digestive system. Those suffering from Celiac Disease have sometimes claimed that they crave blood, or rare cooked meat, for some unknown reason. The body has trouble digesting facts, and gradually starts rejecting dairy products because of underly lactose intolerance. Blood, water, and rare meat are craved because they cause no reactive symptoms, such as diarrhea, vomiting, or stomach cramps.
Anemia - Anemia is a classic "symptom" of a vampire bite. Someone suffering from anemia would gradually become paler, and perhaps need a blood transfusion to help duplicate blood cells. Anemia literally means in its Greek origins "lack of blood", --those suffering from anemia could easily have been suspected to be under the influence of a vampire, or becoming one by superstitious doctors centuries ago.
Migraines - Migraines are associated with vampirism because many people who suffer frequent, or even infrequent migraines seem to agree that before the onset of the intense headache, they are particularly sensitive to sunlight, and other bright lights. Most people suffering from chronic migraines before they were diagnosed would have been repelled by bright sunlight, and this may have aroused suspicion in neighbors and friends, perhaps leading the sufferer to be labeled as a vampire, or even hunted.
Ashley Harness writes for Vampires.com, Werewolves.com, and other sites in the Darksites Network. She\'s involved in several seedy and disreputable activities, smokes too much, and spends her late nights procrastinating for work on her first novel.