Having fangs, drinking blood, and being sensitive to sunlight. Hating garlic, avoiding mirrors, and fearing crosses. These are six defining characteristics of the vampire. But where did these characteristics come from?

The vampire myth has been around in various forms for thousands of years. Stories of vampirism have been found in early cultures in Babylonia, China, and South America. Belief in vampires was especially common in Greece in the 1800s. But despite these early tales, the place most people think of when it comes to vampires is Romania.

Dracula, the 1897 book written by Bram Stoker, is perhaps the most famous vampire story. The tale starts with a lawyer from London who travels to Transylvania (now part of Romania) to help Count Dracula. He is held prisoner by the count, who turns out to be a vampire. Throughout the novel, Bram Stoker describes the vampire’s characteristics. Where did he come up with these qualities?

One hypothesis is that the vampire myth is based partly on the disease of porphyria. Sometimes called the “vampire disease,” porphyria is a blood disorder that was common among the royalty of Eastern Europe, including Romania. It is a genetic disease that causes the body to make less of the protein in red blood cells.

Symptoms of porphyria are surprisingly similar to characteristics of the vampire. The high sulfur content of garlic can bring on an attack of the disease. Repeated attacks cause a patient’s gums to recede, making their teeth look like fangs. Patients also have sensitivity to sunlight, which causes facial disfigurement. And wouldn’t someone with a disfigured face want to avoid mirrors?

They might also want to avoid crosses. During the Spanish Inquisition, over 600 vampires were reportedly burned at the stake by Catholic priests. Some of these victims may have been porphyria patients. After this occurrence, it makes sense that they would not trust Catholic symbols such as the cross.

What about drinking blood? One theory is that because the urine of people with porphyria looks red, they were thought to drink blood. Another theory is that they were advised to consume animal blood because of the problem with their red blood cells.

Could vampire legends be connected to porphyria? Perhaps. When writing Dracula, Bram Stoker was also said to be influenced by the history of King Vlad III of Romania, stories of the Irish potato famine, and his own vivid dreams. Each version of the vampire tale before and after Dracula was almost certainly influenced by multiple factors as well. Like any good myth, the story of the vampire becomes stronger with each retelling, combining fact and fiction to create one of the best horror legends of all time.