Porphyria is a rare hereditary blood disease with symptoms that are uncannily similar to what we now perceive as the classic characteristics of a vampire. People with this disorder cannot produce heme, an essential component of red blood. This makes them extremely sensitive to sunlight, develop sores and scars that do not heal properly, grow excessive amounts of hair, become allergic to garlic (stimulates heme production and can turn a mild case of porphryia into a severe and painful one) and have tightened skin around the lips and gums, thus making the incisors more conspicuous.

Porphyria was relatively unknown as a disease until about the mid-twentieth century, but it is now treatable using regular injections of heme. Certain forms of porphyria are also associated with neurological symptoms, which can create psychiatric disorders. However, the hypotheses that porphyria sufferers crave the heme in human blood, or that the consumption of blood might ease the symptoms of porphyria, are based on a severe misunderstanding of the disease.

There is no real evidence to suggest that porphyria had anything to do with the development of the original folklore, as the hypothesis is mainly based off the characteristics of the modern vampire in any case.

Others argue that there might be a relationship between vampirism and rabies, since people suffering from this disease would avoid sunlight and looking into mirrors and would froth at the mouth. This froth could sometimes look like blood, being red in color. However, like porphyria, there is little evidence to prove any links between vampires and rabies.