Is Cleanliness Making Us Sick

Written by David Roberts, MPH

Key Takeaways

What is the hygiene hypothesis?

The hygiene hypothesis proposes that reduced exposure to microbes in early childhood leaves the immune system poorly calibrated, and more likely to react to things that pose no threat.

It began with one observation. In 1989, epidemiologist David Strachan reported that hay fever and eczema became less common as the number of children in a household rose. [1]

What makes this finding compelling is which siblings influenced the change. The protective effect tracked with older siblings specifically, not simply with family size. [1]

That pattern pointed at something being transmitted downward through a household. Strachan's proposed explanation was that infections passed between siblings through ordinary unhygienic contact were preventing allergic disease from developing. [2]

He deliberately left the mechanism unspecified, and the decades since have filled it in differently than he first supposed. [2]

Through the 1990s the explanation ran through the Th1 and Th2 arms of the immune response, with the idea that too little early immune stimulation tipped children toward the allergy-prone Th2 side. [2]

By the 2000s attention had moved off specific childhood infections entirely and onto broader microbial exposure: farm environments, livestock, unpasteurized milk, pets, and the sheer diversity of organisms living in a home. [2]

How do microbes train a child's immune system?

They supply the signals that teach the immune system what to ignore.

An immune system with nothing to practice on does not end up weaker. It ends up less discriminating, and a less discriminating immune system is what allergy looks like at the cellular level.

The cells doing that work are regulatory T cells, described in the immunological literature as the regulatory anti-inflammatory arm that censors inappropriate immune responses. [3]

They operate at several stages at once: blocking the initial expansion of a T cell clone, preventing the Th2 polarization behind allergy, regulating antibody class switching, and controlling late effector pathways such as mast cell activation. [3]

Different regulatory T cell populations live in different tissues, which may be why allergic disease presents so differently from one person to the next: skin in one child, airways in another, gut in a third. [3]

Several microbial products drive these cells. Short-chain fatty acids, produced when gut bacteria ferment dietary fiber, enhance regulatory T cells directly. [3]

Bacterial breakdown products of tryptophan promote their differentiation through aryl hydrocarbon receptor signaling, and secondary bile acids turn down pro-inflammatory Th17 cells. [3]

Microbial components also trigger tolerogenic dendritic cells and TGF-beta, setting epigenetic immunoregulatory adjustments that persist well beyond the exposure itself. [3]

Immunologist Graham Rook calls the organisms responsible our Old Friends: microbes humans co-evolved with, acquired from mother and family, from the natural environment, and from the symbiotic microbiota itself. [3]

They are largely not pathogens. That distinction shows that the benefit never depended on children getting sick. [3]

What does the research on farm children show?

Children raised in close contact with farm animals develop dramatically less asthma, and the strongest evidence comes from two communities that share almost everything except how they farm.

The Amish and the Hutterites are both of Central European descent. They have similar genetic ancestry, comparable family structures, similar diets, low rates of smoking, and similar vaccination practices.

The difference is agricultural. The Amish farm on traditional single-family dairy operations where children are around horses and cattle daily. The Hutterites use industrialized communal farming, with homes set well apart from the animals.

That makes the comparison close to a natural experiment, with the confounders that usually plague nutrition and environment research largely held constant.

Researchers studied 60 schoolchildren across the two communities and measured what was in the air of their homes. [4]

A fourfold difference in asthma between two genetically similar populations is a large effect by any standard in this field.

The researchers then went further and tested whether the dust itself was doing the work. Mice given Amish house dust extract showed substantially reduced airway reactivity. Mice given Hutterite dust did not. [4]

That step moves the finding from correlation toward cause. Something present in the air of those homes was actively protective, and it transferred.

A much larger European study reached the same conclusion by a different route. Across the PARSIFAL and GABRIELA cohorts, greater diversity of environmental microbes in a child's home was inversely associated with asthma. [5]

The odds ratios were 0.62 and 0.86 across the two cohorts. What protected these children was not any single organism. It was the range of them. [5]

Which exposures matter most?

Six exposures carry most of the evidence, and they range from things a family cannot change to things it can change this week.

Do antibiotics and sanitizers affect immune development?

Both interrupt the microbial contact that shapes a developing immune system, and the antibiotic side of that is now quantified.

A 2024 analysis pooled six high-quality studies and found a dose-response relationship between infant antibiotic exposure and later childhood asthma. [6]

The adjusted odds ratio for the dose effect was 1.05, meaning roughly 5 percent of additional risk with each further course. Risk climbed with the number of courses rather than switching on at any single one. [6]

The same work modeled what that looks like across a population. Pediatric antibiotic prescribing fell 71.5 percent between 2001 and 2018. [6]

Had prescribing instead held at its 2001 level, the modeling estimates 10,053 additional asthma cases over that period, with at least 80 percent of the excess burden falling on children under 10. [6]

Read the other direction, that is encouraging. Prescribing practices already changed, and the asthma burden moved with them.

Antibiotics remain necessary and sometimes lifesaving. The finding concerns courses given prophylactically and therefore not necessarily needed, which is precisely why prescribing guidelines tightened.

Routine sanitizer use and heavy household disinfection work in the same direction. A home scrubbed of its ordinary microbial background gives a child's immune system less to learn from.

That deficit accumulates across exactly the years when immune tolerance is being established, and those years do not come back.

Rates of allergic and autoimmune disease have climbed across developed countries over the same decades in which household disinfection became routine and family sizes shrank.

Which conditions are linked to reduced microbial exposure?

The clearest evidence is in allergic disease, and asthma is the single most studied endpoint.

This is where the three independent lines of evidence converge: farm comparisons, microbial diversity cohorts, and antibiotic dose-response data all point the same way. [4][5][6]

Strachan's original 1989 analysis examined hay fever and eczema. Both showed the same inverse relationship with household size that started the whole line of inquiry. [1]

Food allergy and allergic rhinitis have been examined under the same framework, and the pattern of rising prevalence in industrialized countries holds across them.

Inflammatory bowel conditions including Crohn's disease and ulcerative colitis have risen alongside the allergic diseases in developed countries, and are frequently discussed in the same context of reduced early-life microbial exposure.

The proposed mechanism is the same across all of them. An immune system with an untrained regulatory response will go on to react to targets it should have learned to tolerate. [3]

Whether that misdirected response lands in the airways, the skin, or the gut lining appears to depend in part on where the relevant regulatory T cell populations sit. [3]

What can families do about it?

Most support for a well-trained immune system costs nothing, and none of it requires a household to become less safe.

Get outdoors, and get into contact with soil and animals. Outdoor activity and time in natural environments are named among the primary routes by which protective microbes are acquired. [7]

Gardens, parks, farms and household pets all count. So does letting a child dig in dirt without immediate intervention.

Eat plants, and eat enough of them. Fiber that reaches the colon intact is fermented into the short-chain fatty acids that enhance regulatory T cells. [3]

This is the most direct dietary lever on immune tolerance currently identified, and it works through bacteria a child already has.

Feed the bacteria already there. Prebiotic foods including onions, garlic, leeks, asparagus and slightly green bananas supply the substrate those bacteria ferment.

Add fermented foods. Yogurt with live cultures, kefir, sauerkraut, kimchi and miso introduce living organisms alongside the fiber that sustains them.

Keep sugar in proportion. A diet weighted heavily toward refined sugar tends to crowd out the plant fiber gut bacteria depend on.

Use pharmaceuticals deliberately. Appropriate antibiotic use is named alongside natural childbirth and breastfeeding as a factor in preserving protective microbial exposure. [7]

In practice that means taking antibiotics when they are indicated, and asking whether they are indicated when it is not obvious.

None of this is a single decision. It is the accumulated shape of a childhood, which is also why small changes made consistently are the ones that count.

Frequently Asked Questions

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