Vampire stories have been popping up for centuries, and some of them are a lot scarier -- and less sparkly -- than others.

The most common stories involve dead people walking the Earth with pale skin, a fear of sunlight, and a thirst for blood … but that myth might actually have some basis in scientific fact. Really.

Most cultures have some sort of vampire-like story. In Chinese folklore, for example, there are evil spirits that drain your life away, and in India, there are legends about demons who used to be people, but weren’t cremated properly. Mainly, these myths probably came from a deep misunderstanding of how diseases spread.

During epidemics like the plague, people were understandably terrified and needed an explanation for the apparent randomness of illness. But the vampire myth that’s caught on in pop culture has very specific symptoms, and scientists believe they might be traced back to actual medical conditions. The first possible connection is a group of disorders called porphyria, which are caused by too much of a molecule in your blood called porphyrin.

Your red blood cells mostly contain a massive protein called hemoglobin, which moves oxygen and carbon dioxide around your body. And in the middle of hemoglobin is a ring of carbon, nitrogen, and hydrogen called a porphyrin ring. Normally, enzymes convert porphyrin into another molecule called a heme group.

But for someone who suffers from porphyria, their body doesn’t have enough of one of those enzymes, so the porphyrin builds up and causes problems. For one, it makes their skin really sensitive to sunlight, and they get painful rashes and blisters… but only when they go outside. The excess porphyrin also builds up in their teeth and mouth, causing red or brown discoloration.

It can even look a bit like they’ve been drinking… well, blood. Today, porphyria is treatable, but back in the middle ages, it wasn’t very well understood, so people could have made up all kinds of stories about it. Most kinds of porphyria are also hereditary conditions, so stories could have easily spread about a group of vampires, or one vampire who infected their family.

Another likely origin for the myth is a condition called pellagra. Pellagra is caused by a dietary deficiency of two things -- niacin, also called vitamin B3, and an amino acid called tryptophan. Normally, your body uses niacin to turn food into energy.

And even if you aren’t getting enough of it in your diet, your body will also create niacin from tryptophan, so everything should still run smoothly. But when you aren’t getting enough of either, things go wrong. Around 300 years ago, corn started to replace wheat as the food staple for a lot of Europeans.

This was great because it grew really well and was cheap, but it also wasn’t very nutritious. See, even though corn contains niacin, it’s not in a form that our bodies can digest. In Mexico, where corn had been farmed for centuries before it came to Europe, the kernels were treated with a mixture of water and the mineral lime before use.

That caused a reaction that released all those healthy vitamins, but it wasn’t a practice Europeans brought home with them. To make things worse, corn also doesn’t contain much tryptophan, so there was nothing for their bodies to convert into niacin. No matter how much corn people ate, they couldn’t get the vitamins they needed, so pellagra started popping up all over the place.

The symptoms of pellagra actually look pretty similar to porphyria. Patients are sensitive to sunlight and get terrible rashes if they go outside. Sometimes, the rash gets so bad that the skin starts to degrade over time, looking pale and papery.

Their mouths also turn red, and their tongues start to swell. People are often left with imprints of their teeth in on their tongues, which could have started the idea that vampires have large canine teeth. Pellagra can also cause neurons to degenerate in the brain, which can lead to severe mental illness and insomnia.

Add in a sunlight allergy, and that might explain why the vampires in stories don’t sleep at night. Now, pellagra and porphyria might explain some symptoms, but the whole “walking dead” thing might have come from something totally different. Some think it comes from a neurological condition called catalepsy.

Like epilepsy, it affects signaling in the central nervous system, . They can’t move at all, and their heart and respiration rates slow down until it looks like they’re actually dead. And since doctors back in the day didn’t have access to sensitive machines to measure whether someone was alive, they often thought someone experiencing a cataleptic seizure had died. And what did they do with dead people?

They buried them. Like you can imagine, if you saw some guy climb out of his coffin, it’d be super hard for him to convince you he was actually alive and not a blood-sucking vampire. Ugh.

That’s going to give me nightmares. All three of these conditions still exist, but thankfully, they’re much more treatable or avoidable now. And at the end of the day, we know they’re definitely not anything to be afraid of.

Thanks for watching this episode of SciShow, and Happy Halloween! If you’d like even more spooky science, you can watch our episode from last year, where we explain some bloody amazing facts about vampire bats.