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What lives on us

Right now, while you read this, eight-legged mites are walking out of your eyelash follicles. Trillions of bacteria are digesting your lunch. A virome you never think about is riding shotgun through your blood.

You are not an individual. You are an ecosystem — roughly half microbe by headcount, and about 150-to-1 microbe by gene count. Tap any claim with a number on it and the source pops up. Every claim below comes from peer-reviewed research.

1. The census

For decades, textbooks said bacteria outnumber human cells ten to one. That was a myth built on a 1972 back-of-the-envelope estimate. In 2016, Ron Milo's group at the Weizmann Institute actually counted, and the real ratio came out to about one to one: roughly 38 trillion bacteria to roughly 30 trillion human cells in a reference adult. The entire microbiome weighs about 0.2 kilograms — less than a can of soda.

38 trillion ≈ 30 trillion

bacteria ≈ human cells — the old "10 to 1" is dead.

But headcount undersells them. In 2010, the MetaHIT consortium sequenced the gut's microbial genes and catalogued 3.3 million non-redundant microbial genes — about 150 times your own ~20,000. You carry a second genome. It just isn't yours.

Tap the map. Every region is a different country.

2. The eye bugs

Meet Demodex folliculorum and Demodex brevis — the face mites. D. folliculorum lives head-down in your hair follicles, two to six to a follicle; D. brevis prefers the sebaceous oil glands. They are translucent, worm-shaped arachnids, about 0.3 millimeters long, and the adults walk on eight legs.

They are almost certainly on you. When scientists scraped noses at citizen-science events, they only found mites 14% of the time — but when Megan Thoemmes' group switched to testing for mite DNA, 100% of adults over 18 carried it. Prevalence climbs with age. Babies are born without them; the running theory is that they travel from mother to child.

Their night life

Sleep cycleThey sleep by day and roam by night — they are photophobic, sensitive to light
DietSebum and dead skin cells
MatingAt the follicle surface, then the female lays eggs back inside
LifespanTwo to three weeks, start to finish
SpeedUp to 8–16 mm per hour — a marathon at their scale

Now the famous "fact" you may have heard: that these mites have no anus, store their waste for life, and explode with poop when they die. For years, researchers believed it. Then a 2022 genome study sequenced D. folliculorum, found the anus under the microscope and the genes for one — and quietly corrected the record: they can poop. Science gets to change its mind. That's the whole point.

Most of the time, they are commensals — harmless roommates. But in high numbers they are linked to blepharitis, an inflammation of the eyelids, where cylindrical dandruff at the lash base is a clinical giveaway, and to rosacea, where mite density runs higher than in clear skin. The pattern you'll see in every section: the tenant isn't the problem. The numbers are.

The same genome study found something wilder: D. folliculorum is shedding genes as it adapts to us, sliding from parasite toward symbiont — a species in the act of becoming permanent.

3. The skin

Your skin is 1.8 square meters of habitat — your body's largest organ, and an ecosystem of folds, oil fields, and deserts. Grice and Segre's 2011 review established the founding map: skin communities are site-specific. Oily zones like the forehead run on Cutibacterium; moist zones like the armpit favor Corynebacterium and Staphylococcus; dry zones like the forearm host a more mixed crowd.

The neighborhood watch

Staphylococcus epidermidis, a skin native, doubles as security: it can suppress invading pathogens on the skin's surface. The same Cutibacterium acnes implicated in acne is a normal resident — disease tracks with particular strains and conditions, not the species itself. This is the first rule of the whole post: pathogen is usually a job description, not a species. A tenant in the wrong place, in the wrong numbers, at the wrong time.

The skin keeps its own peace treaty: the acid mantle, a slightly acidic film, makes the surface inhospitable to many would-be invaders while the natives thrive.

4. The gut

The gut is the big city — most of your 38 trillion live in the colon. And they are not just sitting there. Gut microbes synthesize vitamins you cannot make, including K and B vitamins; they train your immune system — roughly 70% of it sits in the gut-associated lymphoid tissue, learning friend from foe; and the enterochromaffin cells of the gut produce about 95% of the body's serotonin, mostly to drive gut motility. Yes, the "happiness chemical" is overwhelmingly manufactured in your intestines, doing plumbing work.

What happens when the city gets shelled? Antibiotics don't only kill the infection — a landmark 2011 study showed the gut microbiota of volunteers took weeks to recover after ciprofloxacin, and repeated courses left communities that never quite returned to their starting state. The recovery is incomplete and individual: some people's ecosystems bounce back, some don't.

The ultimate repopulation

In 2013, a randomized trial gave donor feces — the whole ecosystem, transplanted — to patients with recurrent Clostridioides difficile infections that antibiotics couldn't clear. After one infusion, 81% were cured; after a second, 94%. Vancomycin alone cured 31%. The trial was stopped early — it would have been unethical to continue.

Ninety-four percent is a number medicine almost never sees. That is how much power the ecosystem holds.

5. Friend, foe, tenant

Here is the honest frame. Most of the trillions are commensals — eating for free, hurting nothing — and many are mutualists: they eat, and they pay rent in vitamins, defense, and digestion. "Pathogen" is mostly a tenant that overgrew, wandered into the wrong neighborhood (harmless in the gut, dangerous in the blood), or met an immune system that had never learned the local dialect.

Which is why the hygiene paradox stings. The idea goes back to 1989, when David Strachan observed that children in larger households had less hay fever — the hygiene hypothesis: immune systems that never meet microbes misbehave. Antibacterial overkill — nuking the natives to get the invaders — can leave the city undefended, and empty lots grow the worst tenants.

The closing thought of the microbiome decade: stop asking whether a microbe is "good" or "bad." Ask where it is, how many of it there are, and what the neighborhood looks like. You are not an individual with some bacteria on you. You are an ecosystem that learned to walk.

Myth check

Five claims. True or false?

6. My take

You started reading this as one person. You are finishing as a city: 38 trillion citizens, a million tiny tenants on your eyelashes alone, and 3.3 million genes working the night shift on your behalf. The miracle isn't that they live on you. It's that they keep you alive while doing it.

This post summarizes peer-reviewed findings for a general audience. It is not medical advice. If you have blepharitis, rosacea, or gut symptoms, see a clinician.

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