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It’s Halloween this week, so we’re bringing the science to the supernatural. And this episode is all about our favorite furry frights: werewolves.

Werewolf stories date back at least to the Ancient Greeks, and they were pretty widespread in Europe. You can find them in Nordic and Celtic mythologies, for example. And we’re still fascinated by werewolves, just ask Team Jacob.

While turning into a wolf is definitely not a thing people do, these myths don’t come from nowhere. The idea that a bite could make someone behave strangely or that people could grow thick fur could have come from real scientific phenomena, specifically, a misunderstanding of certain illnesses. The notion that a bite can turn man into beast could have come from rabies, for example.

Rabies is caused by a virus that infects the central nervous system and salivary glands. And like many were-myths, it’s transmitted by the bite of an infected creature. People infected with the virus become agitated and behave very strangely.

They may hallucinate and suffer from insomnia. Those symptoms arise because the virus harms brain cells and interferes with the chemicals that neurons use to communicate with each other. For example, it can mess with the levels of serotonin, a chemical involved in controlling the sleep cycle, feeling pain, and behaviors like aggression.

In later stages of infection, the virus invades the salivary glands, sometimes causing excessive salivation. And that saliva contains transmissible viruses, so if the person bites someone else, they can pass along the virus. Though, human to human transmission of rabies is rare.

Our teeth aren’t particularly sharp and we rarely bite hard enough to break skin. Mammals with pointy canine teeth are much more efficient. And rabies is a life-threatening illness.

If a patient isn’t treated with antivirals and the vaccine before the virus settles in, infections are almost always fatal. Put yourself in the shoes of a rural farmer in medieval Europe. An oddly-behaving, aggressive dog shows up in your village and bites your neighbor.

Then, your neighbor begins acting strangely, as well. They’re agitated, maybe even aggressive, and foaming at the mouth like that scary dog was, and then they die. Back before we understood what a virus was or how they work, a werewolf was as good an explanation as any for how a bite could make a person change like that, especially if you’d heard rumors about people with wolf-like appearances.

Such rumors could have come from real medical conditions like. Congenital Hypertrichosis terminalis: the excessive growth of pigmented hair due to a genetic abnormality. In the past it was literally called “werewolf syndrome” because people with it grow unusually thick, and somewhat fur-like hair on their face and body.

The condition is extremely rare, though. Doctors have documented less than 100 cases since the middle ages. And because of that, it’s been hard to study.

It took almost two decades to suss out what causes the condition in one well-studied family, for example. In 1984 it was clear they had some kind of mutation on the X chromosome. The location was narrowed to a specific region of the X chromosome in 1995, but it wasn’t until 2011 that researchers figured out exactly what was happening to the DNA.

Turns out, an insertion of DNA in that exact spot alters the expression of a gene called SOX3, which is involved in hair growth. The gene becomes over-expressed and expressed in places where it shouldn't be, leading to extra-thick body hair and hair growth on parts of the body that are usually hairless, like a person’s eyelids. Extra hair isn’t really harmful on its own, though the condition is frequently associated with other genetic abnormalities that can cause health or development problems.

But humans are often unkind to people who look different, so the disorder can have a major psychological toll. People with congenital hypertrichosis used to be called “wolf men” and were often featured at fairs or in circus sideshows. Which is horrible.

But way back before then, their existence could have easily fueled werewolf stories. Some of those stories may have even been started by people who actually believed they were werewolves. People with what psychologists call lycanthropy may not look like wolves, but they think they do.

They insist their body has changed, that their canine teeth have gotten longer and pointier and their body hair has gotten thicker. They may even begin to behave like a wolf, running around on all-fours, howling, and demanding meals of raw meat. That’s because lycanthropy is a kind of Delusional Misidentification Syndrome, a type of psychological condition where a person, well, ‘misidentifies’ things.

Basically, something’s gone awry in the part of their brain that recognizes their physical body, causing them to think they’ve become a wolf. If you didn’t know much about mental illness, if you were living in medieval Europe, for example, it would be very unnerving if someone began behaving like a wolf and insisting they had grown sharp teeth and large pointy ears. It’s not hard to see why you might think they were under the influence of a supernatural entity.

But the actual causes of delusional misidentification syndromes remain a bit of a mystery. They’re rare and usually associated with schizophrenia, major mood disorders, or injuries due to stroke or trauma, especially ones that occur in the right frontal lobe of the brain. But scientists are only just beginning to understand how our brain constructs what we know as our physical self.

In the meantime, doctors can help people with lycanthropy with anti-psychotic medications. Similarly, there are a lot of options like cosmetic hair removal that can help people manage congenital hypertrichosis. So nowadays, we know that conditions like lycanthropy or hypertrichosis which could have spurred werewolf myths are nothing to fear.

But you can still be scared of rabies. That is a virus you really don’t want to mess with. Thanks for watching this episode of SciShow!

If you enjoyed learning about the very real diseases that might have inspired werewolf myths, you might like our episode on why so many cultures have vampire stories. [♪ OUTRO].