Book summary “Never Home Alone”: summary

Introduction

You likely view your home as a clean, sterile refuge from the harsh wilderness outside. Modern cleaning products promise to eliminate every trace of bacteria, fungi, and insects to protect your family from disease. Yet this vision of domestic purity is a dangerous illusion that fails to match biological reality.

Modern indoor environments host a rich, diverse wilderness of microbes, fungi, and arthropods that we must learn to manage through ecological gardening rather than futile attempts at total sterilization.

Rob R. Dunn reveals in Never Home Alone that your house functions as a sprawling ecosystem shared with thousands of species. The book traces this hidden domestic nature from microscopic inhabitants in your plumbing to the insects crawling across your walls. You will discover how this indoor wilderness affects your health and why total eradication creates more problems than it solves.

Consider these questions as you read:

• What creatures actually share your living space?
• Why do sealed indoor environments trigger chronic modern illnesses?
• How can you cultivate a healthy home ecosystem instead of waging war on nature?

1. History of Indoor Biology and Germ Theory

The scientific study of indoor life began when Antony van Leeuwenhoek discovered microscopic organisms, but the subsequent rise of germ theory caused science to declare total war on all household microbes. You can trace this shift from wonder to eradication by looking at how early microscopists viewed your domestic space.

Leeuwenhoek and Pepper Water

The scientific study of bacteria and anaerobic microbes began inside a home when Antony van Leeuwenhoek examined black pepper steeped in water under a single-lens microscope. In 1676, after letting black pepper steep in water for about three weeks to discover why it was spicy, Leeuwenhoek drew a cloudy sample into a glass tube and observed thousands of microscopic animals, marking the first recorded human sighting of bacteria. Three years later, he repeated the experiment in a sealed tube where bacteria consumed all the oxygen and continued to grow, discovering anaerobic bacteria.

Leeuwenhoek walked to the market in Delft, bought black pepper, returned home, and added a third of an ounce of pepper to a teacup filled with water.

The Royal Society initially rejected Leeuwenhoek's microscopic discoveries of microscopic organisms until Robert Hooke and Nehemiah Grew painstakingly replicated the experiments. When Leeuwenhoek reported his findings on pepper water, senior Royal Society members such as Robert Hooke and Nehemiah Grew considered them pure imagination. Hooke initially failed to see anything, but after repeated attempts using better microscopes and rigorous steps, he and other members confirmed Leeuwenhoek's observations, giving birth to the formal study of bacteriology.

Robert Hooke, the acknowledged king of the microscopic and author of Micrographia, grumbled and scoffed at Leeuwenhoek's claims before successfully seeing the microbes on his third attempt.

The War on Microbes

The discovery of disease-causing microbes led to germ theory, which caused science to war on all indoor microorganisms and abandon holistic household ecology.

Once Louis Pasteur and others established that certain microbes cause disease, researchers began viewing all invisible indoor life as a threat. Microbiologists focused on killing pathogens, entomologists on killing household insects, and food scientists on eliminating food-borne illness, completely forgetting the potential of household species to inspire wonder or provide benefits. By 1970, nearly all home studies were focused strictly on pathogens and pests out of tens of thousands identified, while only about 50 bacterial species regularly cause disease.

Leeuwenhoek previously showed that microbes could turn good wine into bad vinegar, correctly imagining that most of the life he saw was harmless, whereas later scientists treated every indoor microbe as an enemy.

2. The Hidden Life of Houses and Infrastructure

Modern homes and water systems host vast, complex communities of extremophiles, fungi, and bacteria rather than sterile surfaces.

Extremophiles in Hot Water Heaters

Thomas Brock discovered Thermus aquaticus in Yellowstone and Icelandic geysers, living at temperatures up to 80 degrees Celsius. Later, researchers found similar thermophilic bacteria inside hot water heaters in homes and laundromats, proving that modern indoor infrastructure provides suitable extreme niches for these organisms.

Thomas Brock tested various indoor locations like coffeemakers and lab taps, eventually finding heat-tolerant bacteria in the hot water tap of Jordan Hall at Indiana University and later in water heaters around Madison, Wisconsin.

The discovery of heat-resistant enzymes from bacteria living in extreme heat unlocked the ability to sequence unculturable DNA. Because standard DNA polymerase enzymes are destroyed by the heat required to separate DNA strands, the heat-stable polymerase extracted from Thermus aquaticus (known as Taq) solved this limitation, enabling the polymerase chain reaction (PCR) used in genetic testing worldwide.

Indoor Microbial Diversity

Modern homes host tens of thousands of bacterial species, forming a diverse wilderness rather than a sterile environment. DNA sequencing of dust samples from homes across the United States revealed vast microbial communities composed of body-shedding detritivores, food-rot bacteria, extremophiles, and wild environmental microbes drifting in from outdoors.

Researchers surveyed forty houses in Raleigh, finding nearly eight thousand kinds of bacteria, and later surveyed one thousand houses across the United States, identifying some eighty thousand kinds of bacteria and archaea.

Heavily scrubbed and sealed artificial environments like the International Space Station lack environmental biodiversity and are dominated entirely by human body bacteria. Studies using DNA sampling on the International Space Station revealed an absence of wild environmental microbes and food-associated species, leaving an ecosystem dominated by skin, fecal, and armpit bacteria shed by the male astronauts on board.

Koichi Wakata swabbed the International Space Station, and Jenna Lang analyzed the samples in Jonathan Eisen's lab at UC Davis, discovering high concentrations of Corynebacterium and Bacillus subtilis.

Showerhead biofilms form dense, stable micro-ecosystems with higher biomass than the incoming tap water. Warm water and stagnation allow bacteria to build thick biofilms of excretions, creating a food web where predatory bacteria, protists, nematodes, and fungi interact and multiply.

An average American showerhead hosts a biofilm containing many trillions of individual organisms layered up to half a millimeter thick.

Municipal water treatment using chlorine or chloramine triggers competitive release, causing resilient mycobacteria to thrive. Disinfectants kill susceptible competing species while leaving behind nutrients, allowing chlorine-tolerant nontuberculous mycobacteria to dominate the treated water systems and showerhead biofilms.

Cleaning an individual Denver showerhead with bleach solution led to a threefold increase in the abundance of a Mycobacterium species.

3. The Biodiversity Hypothesis and Chronic Illness

Unlike infectious diseases of the past, modern chronic inflammatory disorders are more prevalent in affluent regions with advanced public health infrastructure. Diseases such as Crohn's disease, inflammatory bowel disease, asthma, allergies, and multiple sclerosis emerged and spread primarily in wealthy, highly developed areas. In the United States over a twenty-year span, allergies increased by 50 percent and asthma increased by a third.

The dramatic rise of modern chronic inflammatory diseases is driven by a lack of exposure to wild environmental biodiversity and protective microorganisms.

Ecologist Ilkka Hanski and his collaborators proposed that the rise in chronic inflammatory diseases is caused by a lack of exposure to wild nature and biological diversity. Ilkka Hanski won the Crafoord Prize in Bioscience in 2011 for his decades of ecological fieldwork tracking dung beetles and butterflies in the Åland Islands.

The Karelia Natural Experiment

A natural experiment across the Finnish-Russian border in Karelia shows that lifestyle differences and environmental exposure dramatically affect allergy rates. While genetically similar, Russian Karelians live traditionally with close contact to soil, farm animals, and forests, whereas Finnish Karelians live in urbanized, sealed environments. Asthma, hay fever, eczema, and rhinitis are three to ten times more common in Finland than in Russia.

In the Karelia Project, researchers found that hay fever and peanut allergies are nonexistent in Russian Karelia, while common across the border in Finland. Teens living in homes surrounded by a higher diversity of rare native plants host more diverse skin bacteria and experience lower rates of allergies. Individuals with a higher diversity of Gammaproteobacteria, specifically of the genus Acinetobacter, on their skin showed immune systems that produced more immunological peacekeeping compounds.

Traditional farming practices expose children to protective environmental dust and microbes that prevent asthma and reduce inflammatory immune responses. Amish children, who farm with horses and live close to barns, have very low rates of asthma, whereas Hutterite children, who use industrial agriculture, suffer asthma rates of 23 percent. University researchers tested mice with egg proteins; while Hutterite dust worsened allergic responses, Amish bedroom dust nearly completely prevented the allergic response to eggs.

Restoring Plant and Soil Biodiversity

To offset modern indoor isolation and reduce chronic inflammation, you should increase your daily exposure to environmental and plant biodiversity. Since the precise species needed remain under study, increasing overall exposure to plants, soil, and animals improves the probability of encountering the necessary protective microbes.

• Plant a greater diversity of plants outside of your home and interact with those plants.
• Tend them.
• Watch them.
• Take a nap on them.
• Grow a garden and sink your fingers into the soil.
• Get a cow that you keep near your backdoor.

4. Fungi, Drywall, and Indoor Ecosystems

Homes harbor thousands of fungal species preloaded into building materials that primarily trigger allergies and asthma rather than direct toxic poisoning.

By analyzing dust samples from over a thousand houses, researchers identified forty thousand kinds of fungi. This indoor fungal diversity actually exceeds the number of named fungal species in all of North America, revealing a massive gap in human knowledge regarding microscopic life. Sampling door sill dust across homes revealed fungi of the genus Tuber that form tree symbioses and produce truffles mimicking male pig pheromones to spread via wild pigs.

Fungi in Drywall and Spacecraft

Spacecraft provide indoor environments where specialized fungi manage to live, grow, and reproduce over multiple generations, often feeding on metals, plastics, and insulation materials. On the Russian space station Mir, fungi ate the insulation around communication wires, causing short circuits and breaking down the station's link with Earth.

Brand-new drywall comes preloaded with dormant fungal spores, including toxic black mold and human pathogens, before it is ever installed in a home. Birgitte Andersen tested thirteen sheets of brand-new drywall from four Danish hardware stores and found Neosartorya hiratsukae on every single sheet, Chaetomium globosum on 85 percent, and Stachybotrys chartarum on half of the samples.

While toxic mold produces dangerous compounds, its primary health impact in homes comes from triggering allergies and asthma rather than direct poisoning. Stachybotrys chartarum can produce macrocyclic trichothecenes, atranones, and hemolytic proteins that cause severe health issues in lab animals, but documented toxic poisoning in humans is extremely rare.

Asthma Remediation Strategies

Actively remediating water damage and removing mold from homes significantly reduces asthma symptoms in children. Researcher Carolyn Kercsmar led a study where children with symptomatic asthma living in moldy homes were divided into groups; the remediation group experienced halved indoor fungi concentrations and far fewer days of active asthma symptoms.

To manage indoor fungi, you can take concrete physical steps:

• Remove wet wood and drywall
• Replace wet material with new, dry material
• Stop the flow of water into the home
• Make alterations to air-conditioning units
• Clean the filter of your air conditioning unit with a vacuum or by hand washing with soap to avoid spreading accumulated fungi
• Open your windows each time you turn your air conditioner on or keep the AC off and open windows to allow environmental bacteria to enter

5. Arthropods and Household Invaders

Ecologists and evolutionary biologists suffer from farsightedness, studying remote locales while ignoring wildlife and animals immediately around us, including in cities and inside houses. Academic training and ecological surveys explicitly exclude heavily urbanized areas. This creates a systemic bias where scientists possess detailed data on rare wilderness species but lack basic abundance data for urban and indoor species like house sparrows, pigeons, or camel crickets. Scientists have heavily sampled forests surrounding New York City while taking far fewer samples in the city itself and almost none indoors.

Exhaustive sampling reveals that typical human homes shelter hundreds of diverse arthropod species that traveled alongside human migration across millennia.

The Giant Asian Camel Cricket Invasion

Humans have maintained a long, ancient relationship with camel crickets dating back to prehistoric cave-dwelling ancestors. Prehistoric humans encountered camel crickets deep in damp, dark caves along with large predators like cave bears. The basements and cellars of modern homes mimic these dark, humid cave environments, providing ideal habitats for camel crickets. In 1914, three boys discovered the Trois-Frères cave in the French Pyrenees, where a bone fragment bore a unique prehistoric carving of a camel cricket of the genus Troglophilus.

Native North American camel cricket distribution maps were actually distorted by an undetected invasion of giant Asian species. When researchers mapped participant responses about basement camel crickets, the results contradicted traditional distribution patterns of native Ceuthophilus species. Field investigations and photographs revealed that 88 percent of submitted pictures actually showed Diestrammena asynamora and D. japonica, introduced Japanese species that spent decades in greenhouses before quietly spreading into residential homes. Researchers MJ Epps and Nathan LaSala used beer pong cups baited with molasses to trap camel crickets at increasing distances from houses in Raleigh, estimating a staggering 700000000 giant Asian camel crickets living in eastern North American basements.

Collecting Indoor Biodiversity

Exhaustive scientific sampling reveals that typical human homes harbor hundreds of diverse arthropod species rather than just a few pests. When a research team meticulously sampled houses room by room, collecting specimens from windowsills, drains, and behind toilets, they found an astonishing array of life. Far from being empty or containing only a handful of problem pests, homes typically shelter at least one hundred to two hundred species across dozens of families. Matt Bertone and the research team sampled Rob Dunn's house in Raleigh, discovering no fewer than one hundred species of arthropods belonging to 304 families across the broader survey.

Many indoor arthropods are ancient human companions that hitchhiked across continents in stored food, building materials, and human belongings. By comparing modern household collections with archaeological data from ancient homes, researchers established that hundreds of species migrated from the Near East and Africa alongside human migrations. These creatures specialized in indoor environments, adapting to feed on stored grain, waste, and food residues. Archaeologist Eva Panagiotakopulu discovered grain and waste beetles in ancient Egyptian homes dating back to 1350 BCE that match species found in modern Raleigh houses, and a 1650 Boston outhouse contained nineteen species of European household beetles.

Studying the diverse animals in our homes offers a profound aesthetic experience that goes beyond simple beauty into the realm of the sublime. Drawing on philosophical distinctions from Immanuel Kant and biologist Harry Greene, physical features like moth wing scales or spider webs can be appreciated as beautiful. However, understanding that these creatures share an ancient, unbroken historical lineage with human travelers through ancient Egypt, Rome, and Columbus's ships provides a sublime intellectual appreciation of the unknown. The author finds deep intellectual thrill and inspiration in observing the hidden arthropods of the home, comparing it to exploring uncharted trails in a Costa Rican rain forest.

When you want to discover what lives with you, follow these steps:

• Search room to room in the house, inspecting corners, drains, windowsills, light fixtures, and spaces under beds and behind toilets.
• Place every living or dead arthropod found into a vial or jar filled with ethanol.
• Take the jars back to the lab to sort, label, and identify each specimen by examining specific anatomical features.
• Pay attention when you see a species in your home and do not assume it has already been studied.
• Take pictures, make drawings, and use a hand lens and a notebook to record what you observe.
• Keep an arthropod in a terrarium to watch what it feeds upon and how it mates.
• Record what the spiders in the corners of your house catch and send your observations or letters to scientists.

6. Pests, Resistance, and Biological Control

Insects rapidly evolve resistance to new chemical pesticides through natural selection. The more aggressive the chemical attack, the faster the target species evolves resistance.

Aggressive chemical pesticide attacks drive the rapid evolution of resistant pests, while biological control using social spiders and wasps offers sustainable alternatives.

German cockroaches in Corpus Christi, Texas, developed resistance to the wonder pesticide chlordane by 1951. By 1966, they evolved resistance to malathion, diazinon, and fenthion, and later to DDT.

Glucose Aversion in German Cockroaches

Certain populations of German cockroaches evolved an innate genetic aversion to glucose. Their taste neurons perceived glucose as bitter instead of sweet, rendering traditional glucose baits useless.

Social Spiders as Pest Control

Traditional societies successfully utilized social spiders inside homes and animal pens to control disease-carrying flies. You can apply this method directly in your domestic spaces:

• Suspend social spider webs in kitchens by a string attached to a nail.
• Place colonies of social spiders in public places like markets, restaurants, milk barns, and hotel kitchens.
• Incorporate special sticks into the construction of homes to make it easier for spiders to build nests.

Specialized parasitoid wasps and entomopathogenic fungi serve as effective biological control agents against household pests. You can protect your home through targeted biological means:

• Spray spores of the fungus Beauveria bassiana on surfaces in a home to target bed bugs.
• Ask a doctor to test for a case of MRSA if you suspect a spider bite.

7. Bacterial Gardening and Fermentation

Seeding bodies with beneficial bacteria and maintaining traditional fermented foods establishes a protective microbial feedback loop against dangerous pathogens.

Heinz Eichenwald and Henry Shinefield discovered that babies handled by a nurse carrying Staphylococcus aureus strain 80/81 during their first twenty-four hours were colonized by it, whereas older babies resisted it due to prior colonization by harmless bacteria. They tested using a benign strain called 502A to occupy resources and produce bacteriocins, preventing pathogens from gaining a foothold through exploitative and interference competition.

At Presbyterian Weill Cornell Hospital and later in Cincinnati, Texas, Georgia, and Louisiana, doctors inoculated newborn babies' noses and umbilical stumps with Staphylococcus aureus 502A, dramatically lowering rates of 80/81 colonization compared to uninoculated control groups.

Charles Elton hypothesized that diverse biological communities occupy available breeding sites, food sources, and cover, making it harder for invaders to establish themselves. Dutch ecologist Jan Dirk van Elsas tested this in the lab by adding nonpathogenic E. coli to sterilized soil with varying numbers of bacterial strains.

Jan Dirk van Elsas filled flasks with soil containing five, twenty, a hundred, or thousands of bacterial strains, showing that E. coli disappeared much faster and struggled to persist in increasingly diverse communities due to resource scarcity and intense competition.

Instead of adopting ecological gardening or rewilding, hospitals chose the eradication approach using synthetic antibiotics like methicillin. Because antibiotic resistance carries an energy cost for bacteria, resistant strains are poor competitors and fail in diverse environments. However, in hospitals where antibiotics constantly kill competitors, resistant strains flourish.

Michael Baym, Roy Kishony, and colleagues at Harvard Medical School used a large rectangular agar plate with increasing concentrations of antibiotics from edge to center, filming bacterial strains mutate and conquer the entire plate in just eleven days.

Bacterial Interference in Newborns

Fermented foods naturally ward off pathogens because the metabolic by-products of fermentation create an environment where most fast-growing pathogenic microbes cannot survive. During fermentation, microbes like Lactobacillus break down sugars to produce acid, alcohol, or gas. Because pathogens generally require fast growth and cannot tolerate extreme pH levels or alcohol, fermented foods effectively weed themselves and act as self-regulating ecosystems.

During the historical cholera epidemic in London, people who drank beer tended to survive better than those who drank water because the alcohol made the liquid safer.

In Korean culture, the concept of sson mhat refers to the specific flavor given to food by the maker's hands. When making kimchi, human skin and gut microbes such as Lactobacillus and Staphylococcus mix into the ingredients and contribute distinct enzymes, proteins, and aromatic profiles to the final product.

Soo Hee Kwon taught cooking classes where participants used identical ingredients and followed the exact same steps, yet every person's finished kimchi tasted different, varying in sweetness, acidity, and aroma.

The Sourdough Experiment and Terroir

Rob Dunn and Anne Madden partnered with the Puratos Center for Bread Flavour to send identical ingredients to fifteen bakers across fourteen countries in 2017. DNA sequencing revealed that the resulting starters contained species matching both the flour and the specific bodies and bakeries of the participating bakers.

A baker renowned for having an unusual fungus named Wickerhamomyces in his starter also had that exact fungus present on his hands, and it was uniquely found only in the starter he produced during the experiment.

While standard hand washing effectively removes transient pathogens without sterilizing the skin's resident microbiome, individuals who spend extensive time handling flour and sourdough develop unique hand microbiomes heavily dominated by Lactobacillus and yeasts like Saccharomyces.

Noah Fierer's previous studies showed that normal human hands typically carry very low levels of Lactobacillus, whereas the bakers' hands in the study showed between 25 and 80 percent Lactobacillus and related species.

You can engage with these biological systems directly:

• Mix simple ingredients like flour and water in a vessel to create a starter.
• Feed the starter repeatedly with water and flour until a stable, acidic, bubbly community forms.
• Continue washing hands with ordinary bar soap and water for at least twenty seconds to remove newly arrived pathogens.
• Maintain low concentrations of pathogens in drinking water.
• Practice effective hand washing to control pathogen spread person to person.
• Ensure everyone is vaccinated for pathogens where vaccines exist.
• Use antibiotics to treat bacterial infections when they arise.

10 Key Ideas

1. Modern indoor environments host a rich, diverse wilderness of microbes, fungi, and arthropods that we must learn to manage through ecological gardening rather than futile attempts at total sterilization.
2. The discovery of disease-causing microbes led to germ theory, which caused science to war on all indoor microorganisms and abandon holistic household ecology.
3. Modern homes and water systems host vast, complex communities of extremophiles, fungi, and bacteria rather than sterile surfaces.
4. The dramatic rise of modern chronic inflammatory diseases is driven by a lack of exposure to wild environmental biodiversity and protective microorganisms.
5. Homes harbor thousands of fungal species preloaded into building materials that primarily trigger allergies and asthma rather than direct toxic poisoning.
6. Exhaustive sampling reveals that typical human homes shelter hundreds of diverse arthropod species that traveled alongside human migration across millennia.
7. Aggressive chemical pesticide attacks drive the rapid evolution of resistant pests, while biological control using social spiders and wasps offers sustainable alternatives.
8. Seeding bodies with beneficial bacteria and maintaining traditional fermented foods establishes a protective microbial feedback loop against dangerous pathogens.