It might give you the creepy-crawlies, but you almost certainly have tiny mites living in the pores of your face right now.
They're known as Demodex or eyelash mites, and just about every adult human alive has a population living on them.
The mostly transparent critters are too small to see with the naked eye. At about 0.3 millimeters long, it would would take about five adult face mites laid end to end to stretch across the head of a pin.
"They look like kind of like stubby little worms," says Michelle Trautwein, an entomologist at the California Academy of Sciences in San Francisco.
Demodex face mites got their name from the Greek words for "fat" and "boring worm," but they're not really worms at all. They're actually arachnids — related to ticks and, more distantly, to spiders.
Trautwein studies our relationship with these microscopic stowaways by looking at their DNA. Her findings suggest that people in different parts of the world have different face mites. "They tell a story of your own ancestry and also a story of more ancient human history and migration," she says.
But before she could tell that story, she needed to find the mites.
"We use a little spoon and scrape it across the kind of greasier parts of someone's face, which isn't as bad as it sounds," Trautwein says.
Once the samples have been collected, she takes them to the lab to look at the genetics.
Trautwein has tested more than 2,000 people, including tourists from all around the world that make their way to the California Academy of Sciences. And she's found DNA evidence of face mites on every single one of them.
"No one is thrilled at the initial notion that they have arachnids on their face," Trautwein says. "But people are often curious — even in their revulsion."
But how could these creatures live on so many people and still go unnoticed?
Our skin is mostly covered by a thin layer of peach-fuzz called vellus hair, with a few notable exceptions such as the palms of our hands and feet. The shaft of each one of those tiny hairs grows out of its own follicle.
Face mites — Demodex folliculorum and Demodex brevis — spend their days facedown inside your hair follicles, nestled up against the hair shaft, where you can't see them.
They eat sebum, the greasy oil your skin makes to protect itself and keep it from drying out. The sebum is produced in sebaceous glands, which empty into the hair follicles and coat both the hair shaft and face mite.
That's why the greasiest parts of your body, such as around the eyes, nose and mouth, likely harbor a higher concentration of mites than other areas.
The mites live for about two weeks. They spend most of their time tucked inside the pores, but while people sleep, they crawl out onto the skin's surface to mate and then head back to lay their eggs.
Since they live inside your pores, you can't scrub them off by washing. It's basically impossible to get rid of all of your face mites.
So how does Trautwein find and study a particular mite? With glue.
"I actually put glue on a glass microscope slide and stick it onto a person's forehead," she says. "Then I slowly peel it off. I look under a microscope for mites that are stuck in the follicles that stick up from the thin layer of skin that got peeled off."
"It can be pretty addictive and exciting," she adds. "It's sort of a meditative process of looking through this microforest of follicles and hairs and looking for just the right potential movement or shape."
It seems our immune system is able to keep their numbers in check, but some people can experience problems with the mites.
"When you tell patients that they have face mites, first of all, they freak out," says Kanade Shinkai, a dermatologist at the University of California, San Francisco.
Shinkai occasionally treats patients who have an overload of face mites, which results in a condition called demodicosis.
"There is a very particular look to people suffering from demodicosis. We call it the Demodex frost," she says. "It's sort of a white sheen on the skin. And if you look really closely, you can see [it] coming out of every pore. If you scrape those pores, you can see it frothing with little Demodex face mites."
The condition is relatively rare and is often connected to a decline in the immune system, such as receiving immunosuppressive drugs after transplant surgery, chemotherapy or immunodeficiency diseases such as AIDS.
Demodicosis can also be triggered by local suppression of the immune system, like using itch-relieving hydrocortisone cream on the face.
It usually comes on fast. "Patients almost universally describe this explosive development of like pustules like whiteheads on their face. It's really dramatic," Shinkai says. "And what's really dramatic about it is that they're often fine the day before, and then they develop it overnight."
For the vast majority of people, though, face mites are nothing to worry about. While some studies have found loose connections between Demodex and diseases like rosacea, the evidence hasn't shown a strong link.
"What's really confusing is that if you go into your office and scrape everyone's face, you would find Demodex probably on everybody," Shinkai says. "And people who have low burden of Demodex may have no or very severe disease and vice versa."
Trautwein also sees face mites more as a source of interest than of fear.
"They're not dangerous in a broad sense because we all have them and most of us seem to be cohabiting quite well with them," Trautwein says. "We mostly share them within family units, and it seems like you are probably initially colonized soon after birth, most likely by your mother, traditionally speaking in human history."
Looking at your mites, researchers such as Trautwein can usually tell something about your geographical ancestry — what part of the world your ancestors came from.
"Face mites are definitely the species of animal that we have the closest connection with as humans, even though most of us don't know about them or ever see one in our lifetime," she says. "We still have this very ancient and intimate relationship, and it seems clear that we've had these face mite species with us for all of our history. So they are as old as our species, as old as Homo sapiens."