Selfsense

Guides · First draft, written October 29, 2026. Facts are still being checked. No dosing.

A beginner's guide to getting to know the non-human parts of you

Here's a slightly unsettling way to start: you aren't the only one living in your body. You've got about as many bacterial cells as human ones, roughly 8% of your DNA came from ancient viruses, and almost every adult has tiny mites living on their face. Don't panic, plenty of these guests are the reason you work at all. This is a guide to who's in there and what they're up to.

How we used to think about a body

For most of the history of biology, an animal was a self-contained thing: one genome, one body, one family line. Anything foreign was either food or a threat. Germ theory backed that up. The immune system was described as a guard telling "self" from "non-self", and medicine's job was to keep the door shut.

Over the last sixty years that picture has been taken apart piece by piece, and most of the pieces were found by people who weren't believed at first.

  1. 1967Lynn Margulis argues that mitochondria and chloroplasts were once free-living bacteria. Many journals turn her paper down before one publishes it.
  2. 1972The phrase "junk DNA" is coined for the big parts of the genome that don't code for proteins.
  3. 1977Carl Woese identifies the archaea as a third domain of life, a whole branch alongside bacteria and the cells of plants and animals.
  4. 1989The "hygiene hypothesis" proposes that allergies rise when children meet fewer microbes.
  5. 1991Margulis coins "holobiont", a host plus all its symbionts, treated as one unit.
  6. 2001The human genome is published. We turn out to have about 20,000 protein-coding genes, not the 100,000 many expected, and those genes take up under 2% of our DNA.
  7. 2010The Neanderthal genome shows that most living people carry Neanderthal DNA.
  8. 2012Biologists publish a paper titled "We have never been individuals". The title becomes a motto for the field.

These days a human is treated less as a single organism and more as an ecosystem with a human in the middle. It's a useful way to think about it, and it's also easy to overdo, so I've tried to keep both in mind below.

The two big ones

Two of these partnerships are big enough to get their own guides.

Guide · mitochondria The guest that stayed How a bacterium swallowed two billion years ago became the power supply of every complex cell, how it ages, and what helps it. Guide · microbiome Thirty-eight trillion neighbours The gut community rebuilt in each of us from birth: what it does, how the thinking changed, and what shapes it.

They're very different arrangements. Mitochondria moved in for good. They can't live outside a cell anymore, and most of their genes now sit in your nucleus. Your gut microbes are more like tenants. You collect a fresh set each lifetime, from your mother, your food and your surroundings, and they change with how you live. Everything else below sits somewhere in between.

The viruses in your DNA

Some viruses, when they infect a cell, paste a copy of their genes into its DNA. If that happens in an egg or sperm cell, the copy gets passed down, and over millions of years it can spread through a whole species. Your genome is full of these fossils.

Most of it is broken and silent. Some of it got put to work, in places you wouldn't guess.

  • The placenta. The cells that fuse to form the outer layer of the human placenta do it with a protein called syncytin, and syncytin is a recycled virus coat protein. Viruses originally used it to fuse with the cells they infected. Without it, a human pregnancy can't work. Even better, other groups of mammals picked up different viruses for the same job, independently.
  • Memory. A gene called Arc, which nerve cells need to form long-term memories, comes from an ancient virus-like element. Its protein still builds hollow shells that look like a virus and carry genetic messages between neurons. So part of how you remember things is borrowed from a virus.
  • The immune system. The enzymes that shuffle gene segments to make billions of different antibodies, RAG1 and RAG2, appear to descend from a "jumping gene" that used to cut and paste itself around the genome. Your defence against infection was built out of a genetic parasite.

"Junk DNA" hasn't aged well as a name. Much of the genome probably does nothing useful, but enough of it does that biologists are a lot more careful about it now.

The viruses living in you right now

On top of the fossil viruses, you carry a live crowd. Most of it isn't interested in you at all.

  • Phages. The most common viruses in your gut infect bacteria, not you. They help decide which bacteria thrive. Before antibiotics, doctors used phages to treat infections, and the idea is back for bacteria that resist every drug. In one well-known case, a man near death from a resistant infection recovered after an emergency phage treatment.
  • Lifelong passengers. Most adults carry several herpesviruses for life, asleep most of the time. One of them, Epstein–Barr virus, infects the large majority of adults, and a large study found that infection with it greatly raises the risk of developing multiple sclerosis later on. That's changing how researchers think about MS.
  • Harmless strangers. Small viruses called anelloviruses float around in most people's blood without causing any known disease. Nobody's quite sure what they're doing there.

Who's on your skin

Your skin has its own bacteria and fungi, sorted by neighbourhood: oily (face, back), moist (armpits, groin) and dry (forearms). They help keep harmful microbes out and tune the skin's immune responses.

Meet the mites

Almost every adult has microscopic mites living in the hair follicles of their face. Demodex mites eat skin oil, come out at night to mate, and get passed on through close contact, mostly from parents. Their DNA even tracks human migration around the world. A 2022 genome study found they're losing genes and becoming entirely dependent on us, the way early mitochondria once did. (The same study cleared up an old belief: they do have an anus after all.)

Your mouth and your blood pressure

Your mouth has its own microbiome, separate from your gut's, and one of its jobs links straight to your circulation. Bacteria on your tongue turn nitrate from leafy greens and beetroot into nitrite, which your body can turn into nitric oxide, a molecule that relaxes blood vessels. In small studies, strong antiseptic mouthwash wiped out that conversion and blood pressure went up slightly. It's an early finding, but I like it as an example of a microbe doing a job we assumed the body did on its own.

Old friends

For nearly all of human history, people carried parasitic worms. Sorry. The "old friends" hypothesis, a refinement of the hygiene hypothesis, suggests the immune system evolved expecting those worms and certain microbes, and partly relies on them to stay calm. Without them it may overreact, which could help explain rising rates of allergies and autoimmune disease in industrialised countries. Researchers have tried deliberately giving worm eggs to people with Crohn's disease and multiple sclerosis. Early results were encouraging, and larger trials mostly weren't, which is a pattern you'll see a lot in this field.

The gut also hosts archaea (some of which make methane), fungi and single-celled organisms. They're far less studied than gut bacteria, and what they do for your health is mostly unknown.

The other humans in you

Not every non-you part of you is a different species.

  • Neanderthals and Denisovans. If you have ancestry from outside Africa, roughly 1–2% of your DNA is Neanderthal, and some populations carry Denisovan DNA too. These leftovers do things. A Denisovan gene variant helps Tibetans live at high altitude, and a stretch of Neanderthal DNA on one chromosome was linked to a higher risk of severe COVID-19. Svante Pääbo, the geneticist who sequenced the Neanderthal genome, won a Nobel Prize in 2022.
  • Your mother, and your children. During pregnancy, cells cross the placenta in both directions. Some of the baby's cells settle in the mother's body and stick around for decades, including, in some studies, in the brain. Most people also carry a few cells from their own mother. It's called microchimerism, and whether it helps or harms is still being worked out. So your mom really is always with you, a little.
  • Mitochondrial Eve. Because mitochondria only pass from mother to child, every living person's mitochondrial DNA traces back to one woman who lived in Africa roughly 150,000–200,000 years ago. She wasn't the only woman alive, just the only one whose line of mothers never broke.

You're a crowd, really: some of it borrowed, some of it inherited, and some of it coming and going every day.

So what do you do with all this?

The practical side follows from thinking of your body as an ecosystem instead of a fortress.

  • Feed the partners. Fibre and lots of different plants feed your gut community, and exercise is the strongest signal to grow and renew mitochondria. Both guides go into detail.
  • Go easy with antibiotics and antiseptics. They save lives, and they're also blunt instruments that hit the partners along with the target. That includes everyday antiseptic mouthwash.
  • Expect to be different. Your microbes, your archaic DNA and your mix of viruses aren't the same as anyone else's. That's part of why the same food, drug or supplement works differently in different people, and why measuring your own response beats trusting an average.
  • Be suspicious of simple stories. "Good bacteria" versus "bad bacteria", "detox", and anything promising to rebalance your virome or microbiome usually runs well ahead of the evidence.

Where I could take this next

Each of these could carry its own guide:

  • The viruses in your genomeSyncytin and the placenta, Arc and memory, and what "junk DNA" turned out to be.
  • The viromePhages, phage therapy for drug-resistant infections, and the lifelong herpesviruses, including the Epstein–Barr link to MS.
  • The skin and its mitesHow the skin's microbes defend it, why harsh cleansing can backfire, and the strange life of Demodex.
  • The mouthOral bacteria, gum disease and heart health, and the nitrate pathway.
  • Your Neanderthal inheritanceWhat archaic DNA does in modern bodies, from altitude to immunity.
  • MicrochimerismThe cells we carry from our mothers and our children.
  • Old friendsThe hygiene hypothesis, worms, farms, and why allergies rose.

This guide explains the science in general terms and isn't medical advice. Several findings here come from small or early studies, and I've said so where it matters.