A woodcut illustration of the totally scientific "C.Lycosthenes" from a 16th-century book by the suspiciously named Konrad Lykosthenes. Wellcome Trust, Wikimedia Commons // CC BY 4.0

Mythical monsters and epidemic disease have long been intertwined. For hundreds of years before germ theory, vampires and other mythical creatures were routinely blamed for outbreaks of disease. Now, thanks to decades of research and a better understanding of basic biology, we’ve reversed the cause and the effect in our monster myths. These days, it’s often the disease that makes the monster, not the other way around. 

Werewolves May Have Had a Blood Disorder.

In 1964, London physician Lee Illis published a paper suggesting that the mythical werewolf was really suffering from a rare set of diseases called porphyria in which there are irregularities in the production of heme, a chemical in blood.

Symptoms of porphyria vary, but they can include extreme sensitivity to sunlight leading to blistering so extreme that a person’s fingers and nose fall off, and the skin on their faces is pulled so tautly that their teeth and gums appear large and menacing. It also can turn teeth and urine red, and trigger abnormal hair growth. Anxiety, hallucinations, and psychosis are rarer symptoms.

Or Maybe They Were Infected by Rabies.

Another suggestion is that werewolf legends may have originated in rabies infections. Rabies is a zoonotic disease—spread between animals and people—and wolves and dogs are two animals which may become infected with rabies and then spread the virus to humans by bites. Rabid dogs and wolves, and the aggressive, salivating werewolf show similar symptoms, suggesting rabies-infected animals may have spawned the werewolf myth.

Vampires May Have Had Lyssaviruses, Too ...

Thirteenth-century physician applies verbena to a rabies wound. Wellcome Trust, Wikimedia Commons // CC BY 4.0

Like the werewolf, many symptoms attributed to both rabies and vampirism were similar: aggression, biting, and hypersexuality. (Rabies can, in rare cases, cause sustained erections and uncontrollable ejaculations in infected men.) Muscle spasms cause the lips to retract, exposing the teeth. Both could be spread by bites, though human rabies is not spread person-to-person like vampirism is. And the garlic factor? That could be related to not rabies but porphyria, which causes a heightened sensitivity to foods with high levels of sulfate—including garlic.

Zombies Have Bacterial Infections and Viruses—including Rabies.

Wikimedia Commons // Public Domain

Zombies long predate George Romero's Night of the Living Dead, most notably in Haiti, where the dead were said to be resurrected by a sorcerer to be forever enslaved. And though one anthropologist has suggested the Haitian zombie state was induced by a mix of hallucinogenic plants and a neurotoxin, these zombies reflect West African religious beliefs more than they do biological agents.

When Romero's unsettling creep fest debuted in 1968, a new brand of zombie was re-born: the reanimated corpse with a taste for flesh. (Romero was inspired by the 1954 novel I Am Legend, in which sci-fi writer Richard Matheson imagines bacteria called Bacillus vampiris sparking a vampire apocalypse—and in the process decoupling vampirism from supernaturalism.) 

Reanimated zombies have been caused by both real and fictionalized microbes. In The Walking Dead, an unnamed virus presumably causes a severe infection leading to death. As everyone in the population is already infected with the virus at a low level, it also causes reanimation upon death from any other cause as well. “Rage” or living zombies, also with a taste for human flesh, are similarly caused by infectious diseases, such as a government-modified form of the rabies virus (always with the rabies!) in Romero’s The Crazies, or a variant of “mad cow” disease, an infectious protein that causes the brain to develop holes and turn to swiss cheese, in Zombieland—to name just a few.

Witches Tripped on Fungus.

Abigail Williams accused many of her Salem neighbors of witchcraft, including George Jacobs. He was hanged. Wikimedia Commons // Public Domain

Infection has also been invoked to explain the rise of witch panics, particularly those from infamous Salem, Mass. It’s been suggested that witch legends have a natural cause: ergotism, or a poisoning resulting from a fungus growing on grains of rye.

Symptoms of ergot poisoning can be dramatic: crawling sensations on the skin, dizziness, disturbances in sensation, hallucinations, mania, psychosis, delirium. These symptoms match descriptions in the records of the Salem trials. Weather patterns in the year prior to the “outbreak” of witches show that Salem experienced a warm, rainy spring; perfect conditions for the fungus to grow. Grain was harvested and stored for months, then used in the fall—shortly before the first witchcraft accusations in December 1691. The dry summer of 1692 presumably put an end to the infectious sorcery.

And then there are the monsters in your Head.

Even monsters that are true figments of our imagination may have a biological cause. Apparitions of demons, alien abduction, or succubi can be the result of sleep paralysis, a phenomenon where the sleeper is aware, but unable to move. Most frequently occurring as one is drifting off to sleep or waking up, this condition can lead to vivid and very realistic hallucinations, including the feeling of suffocation frequently associated with demon sex, the paralysis of laying on an alien operating table, or the apparition of a ghost. True “Ghost Hunters” may be better served by spending time in a sleep clinic rather than dilapidated buildings.