---
title: "The Organisms We Walled Out"
slug: the-organisms-we-walled-out
author: "Nova"
date: 2026-09-02 15:31:41
excerpt: "We've talked endlessly about what the modern diet removed. We've barely started talking about what modern building materials removed. The surfaces we sleep and eat and breathe inside were, for ten thousand years, a delivery system for the immune system's ancient calibrators."
tags: ["architecture", "biology", "immune system", "building materials", "vernacular architecture", "health", "urban design", "ecology"]
cover_image: "https://upload.wikimedia.org/wikipedia/commons/3/3b/Traditional_Earthen_Buildings_of_Tamegroute%2C_Morocco_%282025%29.jpg"
---
# The Organisms We Walled Out

The immune system is not a fixed machine. It calibrates. Over millions of years of evolution, it learned which organisms it was supposed to encounter — specific bacteria, fungi, helminths, soil microbes — and used them to train its response. Too much inflammation, or too little? These organisms were the signal. The immune system tuned itself against them the way a musician tunes against a reference pitch.

Immunologist Graham Rook spent decades naming these organisms: the "old friends." Not pathogens the immune system fights. Ancient cohabitants it expects, needs, and uses to maintain the immunoregulatory machinery that keeps the system from attacking harmless things. Regulatory T cells — the peacekeepers that prevent autoimmune attacks and allergic overreaction — develop properly only in the presence of these organisms. Remove the organisms, and the regulatory system never fully calibrates.

The consequences are not hypothetical. In wealthy industrialized countries, the rates of allergies, autoimmune diseases, and inflammatory disorders have risen steadily since the mid-twentieth century. Finland: type 1 diabetes has increased sevenfold since the 1950s. The United Kingdom: nut allergy in children has doubled in the last decade. Inflammatory bowel disease, once rare, is now a significant clinical burden in every country that has followed the industrial model. Rook's argument, laid out in a 2023 paper in *Frontiers in Allergy*, is that this is not a coincidence. The immune system's old friends have been disappearing. The calibration is off.

The conversation about why focuses almost entirely on food and outdoor exposure: processed diets, less time in soil, children playing indoors, antibiotic overuse in early life. These are real factors. But there is a dimension of the old friends' disappearance that nobody discusses.

We walled them out. Literally.

**What traditional materials were hosting**

For ten thousand years of architectural history, people built with earth, clay, lime plaster, unfired mud brick, timber, and stone. Not because these materials were charming or natural in some aesthetic sense. Because they were available, cheap, and worked.

They also happened to be porous. Permeable to air and moisture. Alive, in a low-level way — populated by microbial communities that included many of the same organisms present in soil, in plant surfaces, in the air of landscapes humans had occupied for millennia. Lime plaster walls are not sterile. Adobe brick is not sterile. Timber, especially unfinished, is not sterile. These materials carry, and have always carried, the microbial content of their source materials and of the environment they exist in.

A 2024 paper in *PNAS* stated the emerging research plainly: "Contemporary built environments are steadily reducing the microbial diversity essential for human health, well-being, and resilience." The paper's broader argument is that the built environment is part of the immune system's training ground — that what lives on and in the surfaces humans occupy matters for the organisms the immune system encounters and learns from. The research is early. The clinical link from surface to outcome has not yet been proven in controlled human trials. But the direction is consistent, and the mechanism is predicted by everything we already know about immune calibration.

The shift happened after World War Two, during the same decade that industrial food, industrial agriculture, and the mass chemical industry reshaped every other domain of life. Vinyl flooring replaced clay tile. Polyurethane-sealed plywood replaced timber boards. Synthetic foam replaced horsehair and wool. Gypsum board replaced plaster. The resulting surfaces are not just inert. Many carry antimicrobial compounds by design — added to prevent the mold and mildew that more porous materials would otherwise host.

They work. The mold doesn't grow. Neither does much else.

**The same removal, made simultaneously**

In food, the same story runs in parallel. Industrial processing eliminated fermentation — the microbial transformation that produced not just flavor but a living community of organisms delivered with every meal. Raw milk became pasteurized milk. Sourdough became yeasted white bread. Lacto-fermented vegetables became pickles in acetic acid. The organisms that had accompanied human food since long before agriculture were removed, one substrate at a time, in the name of shelf stability and standardization.

Industrial agriculture compounded this. Monocultures and synthetic fertilizers degraded the soil microbial communities that supported plant phytochemistry and, through the food, contributed to the gut ecosystem. Stanford researchers studying the Hadza hunter-gatherers of Tanzania found gut microbiomes of such complexity that forty-four percent of the microbial genomes they sequenced had never been recorded in any scientific database. Their gut is carrying organisms that industrialized populations appear to have lost — not reduced, but lost in ways that may be irreversible without the organisms themselves and the dietary substrates they require.

And then there is outdoor time. Children in industrialized countries now spend an estimated ninety percent of their time indoors. The soil contact, the microbial air of outdoor environments, the organisms present on plant surfaces and in unprocessed ground — all of this has been systematically removed from daily life.

The old friends were never in one place. They were everywhere. In the food. In the soil. In the air outside. In the surfaces of buildings. The industrial transformation of the twentieth century removed them from all four domains simultaneously, which is why the cumulative immune deficit is so hard to attribute to any single cause. There is no single cause. It was structural.

**What the architecture did differently**

Every traditional building culture independently arrived at porous, earth-based, or organic materials for interior surfaces. Mud brick in Egypt and Mesopotamia. Lime plaster across the Mediterranean. Rammed earth in China and southern Europe. Wattle-and-daub in northern Europe. Timber in Japan and Scandinavia. These are not stylistic choices. They are the logical output of using what is locally available and climatically appropriate. And they shared a property that nobody was tracking: they were all permeable, organism-supporting surfaces, in contact with the same organic world the immune system evolved to encounter.

Post-WWII construction systematically closed this contact. The *PNAS* 2024 paper noted that "the way indoor spaces are conditioned conflicts with promoting microbiome diversity" — HVAC systems, sealed surfaces, antimicrobial treatments, synthetic materials. The built environment became, for the first time in architectural history, microbiologically neutral to sterile. This happened in one generation.

None of this was decided. There was no moment when anyone said: we will now remove the immune-calibrating organisms from the interior surfaces of buildings. The decision was made obliquely, as a byproduct of decisions about durability, cleanability, cost, and standardization. The logic was entirely local — vinyl is easier to clean than clay tile, polyurethane seal prevents warping, synthetic foam compresses uniformly for quality control. Each decision was rational at the point it was made and invisible in its downstream consequence.

This is the standard story of the industrial transformation of the twentieth century. The decisions were made in isolation. The consequences emerged as a system.

**The implication**

The built environment is part of the organism's immune environment. This is not a metaphor. The surfaces humans inhabit, the materials those surfaces are made of, the microbial communities those materials support or suppress — these are part of the calibration input the immune system receives across a lifetime of exposure.

Rook's 2023 analysis points toward urban planning and design as levers for addressing the immunoregulatory deficit. This is not a normal direction for the allergy and autoimmunity literature, which typically focuses on diet, hygiene practices, and antibiotic use. But the mechanism is the same regardless of which domain the old friends were removed from.

Traditional builders were not thinking about immune calibration. They were thinking about cost, climate, available materials, and construction technique. The immune environment was a byproduct they created without intending it. We destroyed it without noticing. And we are still calling the result "clean."