Science & the Human Condition
You Are Only
10% Human
The trillions of microbes living inside you are not passengers — they are co-pilots. And ignoring them may be making you sick.
of the cells in your body are not human.
They are bacteria, fungi, viruses — a vast inner ecosystem we are only beginning to understand.
Consider the last time you looked in a mirror. You saw a human face, human hands, a human body. But modern science reveals a startling truth: if you were to count every cell that makes up what you call "you," the vast majority would not be human at all. For every one human cell, you carry approximately nine microbial cells — bacteria, viruses, fungi, and archaea that colonise your skin, gut, lungs, and mouth. In terms of sheer numbers, you are more microbe than man.
This is the central revelation of biologist Alanna Collen's landmark book, 10% Human: How Your Body's Microbes Hold the Key to Health and Happiness. Published in 2015, it draws on decades of cutting-edge research to argue that the collection of microorganisms living within us — collectively known as the microbiome — plays a far more profound role in our health, behaviour, and identity than we ever imagined.
The Invisible Civilisation Within
The human gut alone houses roughly 100 trillion microorganisms, representing over 1,000 species. Together, these creatures carry approximately 4.4 million unique genes — compared to the mere 23,000 in the human genome. In this sense, our microbial passengers are not an afterthought of biology. They are an essential operating system.
Microbial density by body region
For most of human history, we have treated these microbes as either irrelevant or dangerous. The 20th century's great war on germs — through antibiotics, antiseptics, and aggressive hygiene — was, in many ways, a war on ourselves. We sterilised and sanitised without understanding the delicate ecology we were dismantling.
"We are not just human. We are superorganisms — composite creatures whose health, mood, and behaviour are shaped in equal measure by our genes and by the microbes we carry."
— Alanna Collen, 10% HumanThe Microbiome and Modern Disease
Collen builds a compelling case that the dramatic rise of so-called "21st-century illnesses" — obesity, allergies, autism, anxiety, autoimmune disorders, and even depression — is intimately linked to the disruption of our microbiomes. These conditions were rare or unknown in previous centuries. Their sudden prevalence coincides almost precisely with our increasing use of antibiotics, caesarean births, formula feeding, and ultra-processed diets.
Take antibiotics. A single course can wipe out up to a third of gut bacteria species, and in some cases those species never fully recover. Children who receive antibiotics in the first year of life show markedly higher rates of obesity, asthma, and allergies in later childhood. This is not coincidence — it is ecology. Remove a keystone species from a forest and the whole system shifts.
Obesity offers perhaps the most striking example. Collen cites experiments in which germ-free mice — raised to have no microbiome at all — could eat as much as they liked and remain lean. When the gut bacteria from obese mice were transplanted into these germ-free mice, they gained weight without changing their diet. The microbiome, it seems, is not merely processing food — it is deciding, to a significant degree, how much energy our bodies extract from it.
The Gut–Brain Connection
Perhaps the most astonishing argument in Collen's book concerns the relationship between the microbiome and the brain. The gut contains more than 100 million neurons — so many that scientists call it "the second brain." It communicates with the central nervous system via the vagus nerve in what amounts to a constant, two-way conversation. And the microbes that live in the gut are active participants in that conversation.
Certain gut bacteria produce neurotransmitters — including serotonin, dopamine, and GABA — the same chemicals that regulate mood, anxiety, and cognition. Studies in mice have shown that altering the gut microbiome can transform personality: timid mice become bold, and bold mice become cautious, simply by transplanting gut bacteria between the two groups. The implications for human mental health are staggering.
"The bacteria in your gut may be shaping your thoughts, your moods, and even your personality — more than you or your therapist ever suspected."
— Based on research cited in 10% HumanCollen does not overstate the evidence — she is careful to distinguish what is proven from what is speculative. But the trajectory is clear: a growing body of research links gut microbiome disruption to anxiety, depression, autism spectrum disorder, and even schizophrenia. Our mental health, it appears, is not solely in our heads.
How Modern Life Disrupts the Microbiome
Collen identifies four major ways that contemporary life has impoverished our inner ecosystem. First, caesarean birth: babies delivered by C-section miss the critical bath of maternal vaginal bacteria during passage through the birth canal — bacteria that seed the infant gut and immune system. C-section babies show higher rates of allergies, asthma, and obesity throughout life.
Second, formula feeding: breast milk is not merely nutrition. It contains hundreds of species of bacteria, along with complex sugars called oligosaccharides that serve no purpose for the infant — they exist solely to feed specific beneficial gut microbes. Formula provides none of this microbial inheritance.
Third, the overuse of antibiotics: prescribed too readily for viral infections they cannot treat, antibiotics have become ecological catastrophes at the microscopic scale, wiping out beneficial species alongside harmful ones.
Fourth, and perhaps most pervasively, diet: the Western diet — high in sugar, refined carbohydrates, and ultra-processed food — actively starves the gut microbiome of the dietary fibre it needs. A diverse, plant-rich diet feeds a diverse microbiome. A junk food diet feeds only a narrow range of opportunistic species.
Reclaiming the Inner Ecosystem
The good news, Collen argues, is that the microbiome is not fixed. It is dynamic, responsive, and, to a degree, within our control. Increasing dietary diversity — eating 30 or more different plant foods per week — is one of the most powerful ways to cultivate a healthy gut community. Fermented foods such as yoghurt, kefir, kimchi, and sauerkraut introduce beneficial live cultures directly into the gut.
Faecal microbiota transplantation (FMT) — the transfer of gut bacteria from a healthy donor — has already proved remarkably effective in treating Clostridioides difficile infections, and clinical trials are exploring its potential in conditions from Crohn's disease to depression. The idea of treating illness by rebalancing the microbiome, once dismissed as fringe science, is now the frontier of mainstream medicine.
The Hidden Parasites Science Cannot Name
Here lies perhaps the most unsettling frontier of microbiome research — a question that scientists are only beginning to dare ask: what if some of what we consider "us" is not us at all? Within the vast catalogue of microorganisms living in and on the human body, a significant proportion remain functionally unclassified. They are present. They are active. But whether they are partners, passengers, or quiet predators — science cannot yet say.
The human microbiome contains hundreds of microbial species that have never been successfully cultured in a laboratory. They exist as genetic shadows — detected by DNA sequencing, but invisible to traditional microbiology. Their metabolic roles are unknown. Their interactions with host tissue are unmapped. And crucially, their relationship with the human body — whether mutualistic, commensal, or parasitic — remains entirely open.
The problem is compounded by a profound limitation in how we study the microbiome: most of our knowledge comes from analysing what microbes are present, not what they do. Genomic sequencing can identify a bacterial strain by its DNA, but it cannot easily determine whether that organism is currently harming or helping its host. The biological distinction between a symbiont and a parasite can be vanishingly thin — and context-dependent. A species that behaves beneficially in a healthy host may turn opportunistically parasitic the moment the immune system weakens.
"The line between commensal and parasite is not a wall — it is a gradient. And we do not yet know where on that gradient most of our microbes sit."
— Frontier thinking in microbiome scienceWhen "Self" Becomes Indistinguishable from "Other"
The identity problem runs deeper still. Some microbial organisms have become so thoroughly integrated into human biology that even our most sophisticated immune systems cannot reliably distinguish them from native tissue. These so-called "stealth microbes" evade immune detection not by attacking — but by disguising themselves. They mimic human cell-surface proteins. They co-opt human signalling molecules. They slip beneath the threshold of immune recognition so completely that the body treats them as self.
One striking example is Toxoplasma gondii — a parasitic protozoan estimated to silently inhabit the brains of approximately one-third of all humans on Earth. For decades it was considered dormant and harmless in healthy adults. Yet accumulating research suggests it may subtly alter dopamine pathways, shift risk tolerance, slow reaction times, and potentially increase susceptibility to schizophrenia and bipolar disorder. The parasite does not announce itself. It asks no permission. And for most of its hosts, it passes an entire lifetime without detection.
The Toxoplasma Problem
Toxoplasma gondii infects an estimated 2 billion people worldwide. In most it causes no obvious illness. Yet it embeds permanently in brain and muscle tissue, and growing evidence suggests it quietly reshapes personality, cognition, and mental health — entirely without the host's knowledge or consent. It is, by any definition, a parasite. But it is one that billions of people carry as though it were simply part of themselves.
If Toxoplasma can do this openly — and we only discovered its neurological effects decades after recognising its existence — what might the hundreds of uncharacterised, uncultured microbial species within us be doing in silence? Scientists now suspect that certain chronic inflammatory conditions, autoimmune diseases, and even some psychiatric disorders may be driven, in part, by microorganisms that our diagnostic tools have not yet learned to see as causative agents. The microbe is present. The disease is present. But the causal thread remains invisible.
There is also the matter of the human virome — the vast collection of viruses that colonise the body. Unlike bacteria, most viruses integrated into the human system produce no symptoms and have never been formally classified as pathogens. Yet the virome is enormous. Hundreds of novel viral species have been discovered in healthy human gut samples within the last decade alone. Their function is almost entirely unknown. They may be beneficial. They may be neutral. Or some may be parasitic in ways we simply do not yet have the scientific vocabulary to describe.
"We are walking ecosystems containing multitudes we have not catalogued, governed by rules we have not written, shaped by organisms we cannot yet name."
— The emerging challenge of microbiome scienceThe Philosophical Weight of the Unknown
This uncertainty carries a weight that extends beyond biology into something more fundamental: the question of agency. If a microorganism embedded in your brain is influencing your decisions, your cravings, your personality — and you cannot detect it, cannot remove it, and science cannot yet even confirm its role — in what sense are those decisions entirely yours?
This is not science fiction. It is the logical extension of what microbiome research is now revealing. The ancient philosophical question — where does the self end and the world begin? — has found a microbial answer. The boundary is not the skin. It is not even the cell membrane. In the deepest biological sense, the self is a negotiated territory, partly occupied by organisms whose allegiance remains unknown.
What makes this especially vertiginous is the possibility that some of these parasitic or ambiguous microbes may have been with us so long — co-evolving across millions of years — that our genomes have partially adapted around them. We may have built our biology, in part, to accommodate entities that were never truly on our side. The relationship would not be symbiosis. It would be something stranger: a coexistence so ancient that the body no longer remembers how to function without its uninvited guests.
Do Viruses Even Exist — Or Are They Something Else Entirely?
Here we arrive at the most radical question the microbiome revolution has quietly opened: what if viruses, as we currently define them, are not a distinct category of life at all? What if what we call a "virus" is simply a microbial entity in a particular state — a commensal organism pushed past a threshold, shedding fragments of itself, triggering the very inflammatory cascade we have learned to call "infection"?
This is not mainstream virology. But it is a question that a minority of serious scientists have been circling for decades — and the tools of modern genomics are making it harder, not easier, to dismiss. The challenge begins with the definition of a virus itself. Viruses are conventionally described as non-living, acellular particles: protein shells carrying genetic material, entirely dependent on host cell machinery to replicate. They are portrayed as invaders — external entities that breach the body, hijack its cells, and multiply at the host's expense.
The human genome itself complicates the picture enormously. Approximately 8% of our DNA consists of sequences derived from ancient retroviruses — so-called endogenous retroviruses (ERVs) — that infected our ancestors millions of years ago and became permanently incorporated into the germline. These sequences are now inherited like any other gene. Some have been co-opted for essential human functions: certain ERV proteins are critical to placental development, immune regulation, and even early embryonic cell division. The virus became us. At what point did it stop being a pathogen and become a part of our genome? And more disturbingly — is that process still ongoing?
"Eight percent of the human genome is viral in origin. We did not catch those viruses. We became them. The question is how many of our current 'infections' are simply that process, still in motion."
— A question raised by endogenous retrovirus researchThe Terrain Theory Revisited
In the 19th century, a fierce debate divided the founders of microbiology. Louis Pasteur championed the germ theory — the idea that specific external microorganisms cause specific diseases, and that medicine's task is to identify and destroy them. His contemporary Antoine Béchamp argued instead for terrain theory — that the internal condition of the body, its "terrain," determines whether a microorganism causes disease. The same organism, Béchamp insisted, could be harmless in a healthy host and pathogenic in a disrupted one. Pasteur's germ theory won. It drove the entire architecture of 20th-century medicine — vaccines, antibiotics, antivirals, and the entire framework of infectious disease.
But microbiome science is quietly rehabilitating Béchamp. The concept of pathobiont — a microorganism that is commensal in a healthy host but becomes pathogenic under conditions of dysbiosis or immune disruption — is now firmly established in mainstream microbiology. Helicobacter pylori lives harmlessly in the stomachs of most people who carry it, yet causes ulcers and cancer in a subset. Candida albicans is a permanent resident of the human gut in the majority of adults; it causes life-threatening systemic infection only when the immune balance shifts. These organisms do not change. The terrain does.
Apply this logic to virology, and a deeply unsettling possibility emerges. What if many of the entities we classify as "viruses" are in fact cellular debris, exosomes, or genetic fragments shed by stressed or dying microbial communities within the body — triggered not by an external invader, but by internal collapse? The body, under conditions of toxicity, malnutrition, immune disruption, or severe stress, begins shedding cellular material that, under an electron microscope, looks indistinguishable from what we call a virus. We see particles. We sequence their genetic material. We build a narrative of invasion — when the reality may be one of internal disintegration.
"What if what we call a viral outbreak is, in some cases, not an invasion from without — but a distress signal from within?"
— The terrain theory challenge to modern virologyThe Virome: Permanent Residents or Permanent Suspects?
Modern sequencing has revealed that healthy humans carry a vast and largely uncharacterised collection of viral sequences — the virome — at all times. The gut virome alone contains hundreds of thousands of distinct viral genetic sequences in a single individual. The vast majority have never been associated with any disease. They appear to be permanent, heritable, and stable across generations within families. In short, they look far more like residents than invaders.
The most abundant entities in the human virome are bacteriophages — viruses that infect bacteria. They act as a regulatory layer within the microbiome, culling bacterial populations, transferring genes between species, and potentially shaping the microbial ecology of the gut with profound effects on human health. When the virome destabilises — as it does in conditions like Crohn's disease, HIV, and severe COVID-19 — it may be not a cause of disease but a reflection of a microbiome already in collapse. The viruses, on this reading, are not causing the fire. They are the smoke.
Koch's Postulates — The Unsatisfied Standard
To formally prove a microorganism causes a disease, it must satisfy Koch's Postulates: be found in all cases of the disease; be isolated from the diseased host; cause the disease when introduced to a healthy host; and be re-isolated from that host. For several major viral illnesses — including some that shaped 20th-century medicine — these postulates have never been fully satisfied. The particles are found. The disease is present. But the causal chain, in strict scientific terms, remains an inference, not a proof. In an era where microbiome science is rewriting causality at the microscopic scale, that gap deserves scrutiny.
None of this is to say that viruses do not cause harm. The evidence for viral pathogenicity in many conditions is overwhelming, and to dismiss it entirely would be intellectually reckless. But the binary we have constructed — clean, external virus versus healthy, internal body — is almost certainly too simple. The reality is a continuum: a vast, dynamic community of genetic entities, some ancient residents of the human body, some genuine external agents, and many occupying a territory between those categories that our current science cannot yet map. The virus may be, in many cases, less a discrete organism than a state — a mode that genetic material enters when the conditions of the host demand it.
What that means for medicine, for public health, and for our understanding of what it means to be sick — and what it means to be human — is a question that the next century of science will have to answer.
Ultimately, 10% Human asks us to revise a fundamental assumption: that we are discrete, self-contained individuals. We are not. We are ecosystems. The boundary between "self" and "other" dissolves when you understand that the microbes within you are not merely passengers — they digest your food, train your immune system, regulate your hormones, and converse with your brain.
To care for your health is to care for your microbiome. To understand yourself is to understand the invisible civilisation that makes you who you are. And perhaps most humbling of all: the qualities you consider most uniquely human — your mood, your appetite, even certain aspects of your personality — may be shaped, in ways we are only beginning to fathom, by organisms that are not human at all.
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