When German Scientists Tried to Rename Bats and Shrews, Hitler Threatened to Send Them to War

iStock // Heinrich Hoffmann/Keystone Features/Getty Images // collage by Jen Pinkowski
iStock // Heinrich Hoffmann/Keystone Features/Getty Images // collage by Jen Pinkowski

In The Art of Naming (The MIT Press), Michael Ohl, a biologist at the Natural History Museum of Berlin, delves into the art, science, language, and history of taxonomy. There are some 1.8 million known species—and scientists estimate that 100 million more await discovery. Every one will need a name. How does the process work? 

Ohl takes us into the field with the explorers and scientists at the forefront of naming the natural world, including Father Armand David, a French priest who was the first to describe the panda to the Western world; American paleontologists Edward Dinker Cope and Othniel Charles Marsh, who bitterly battled in the Bone Wars; and Polish biologist Benedykt Dybowski, whose unique naming system for crustaceans called gammarids (a.k.a. "scuds") resulted in tongue-twisters such as Cancelloidokytodermogammarus (Loveninsuskytodermogammarus) loveni.

In the excerpt below, Ohl tells the story of one of the little-known footnotes to World War II: When Adolf Hitler threatened the German biologists who wanted to rename bats and shrews. And, read on for the best bat nickname of all time: "bacon mouse."

—Jen Pinkowski

***

On March 3, 1942, a brief item with a rather peculiar headline appeared tucked away in the Berliner Morgenpost newspaper. "Fledermaus No Longer!" the bold letters proclaimed. The following short text was printed underneath:

"At its 15th General Assembly, the German Society for Mammalogy passed a resolution to change the zoologically misleading names 'Spitzmaus' [shrew] and 'Fledermaus' [bat] to 'Spitzer' and 'Fleder.' Fleder is an old form for Flatterer [one that flutters]. The Spitzmaus, as it happens, has borne a variety of names: Spitzer [one that is pointed], Spitzlein, Spitzwicht, Spitzling. Over the course of the conference, several important lectures were held in the auditorium of the Zoologisches Museum […]."

To this day, despite the problems announced by Germany's leading specialists on mammals on the pages of one of the capital's daily papers, fledermaus and spitzmaus remain the common German names for bats and shrews. Neither dictionaries nor specialized nature guides contain entries for fleder or spitzer (provided one disregards the primary definition of spitzer, which is a "small implement used for the sharpening of pencils").

Indeed, a swift response to the item in question arrived from an unexpected source. Martin Bormann, Adolf Hitler's private secretary, sent a message on March 4, 1942, to Hans Heinrich Lammers, head of the Reich Chancellery. The missive contained remarkably unambiguous instructions from Hitler:

"In yesterday's newspapers, the Führer read an item regarding the changes of name ratified by the German Society for Mammalogy on the occasion of its 15th General Assembly. The Führer subsequently instructed me to communicate to the responsible parties, in no uncertain terms, that these changes of name are to be reversed immediately. Should members of the Society for Mammalogy have nothing more essential to the war effort or smarter to do, perhaps an extended stint in the construction battalion on the Russian front could be arranged. Should such asinine renamings occur once more, the Führer will unquestionably take appropriate measures; under no circumstance should terms that have become established over the course of many years be altered in this fashion."

There's no question that the "responsible parties" understood and responded to the injunction, which could hardly have been misinterpreted. On July 1, 1942, at least, a notice was printed in the Zoologischer Anzeiger—at that time, the "organ of the German Zoological Society"—that comprised a scant five lines. The notice has no byline and can most likely be attributed to the journal's publishers:

"Regarding the discussion [in earlier issues of the Zoologischer Anzeiger] about potential changes to the names 'Fledermaus' and 'Spitzmaus,' the Editors wish to make public that terms that have become established over the course of many years are not to be altered, following an announcement by the Reich Minister of Science, Education, and National Culture, as per the Führer's directive."

It's conceivable that Lammers forwarded Hitler's instructions (which had reached him by way of Bormann) to Bernhard Rust, the Reich Minister of Science, Education, and National Culture. Rust will then likely have ordered one of the "parties responsible" for the unpopular initiative to publish the retraction in the appropriate platform. The Zoologischer Anzeiger fit the bill, considering the fact that by 1941 it had already featured two articles debating whether the name spitzmaus should be changed.

What is the problem, though, that veteran scientists have with spitzmaus and fledermaus, those innocuous terms for the shrew and the bat? And how could it come to pass that Adolf Hitler—preoccupied as he was in 1942— should personally join in the campaign for the correct classification of these small mammals?

 
 

The common thread in these two unremarkable and familiar terms is of course the second word component, maus, or "mouse."

Fledermaus and spitzmaus … are (linguistically) first and foremost mice. By referencing certain characteristics in these compound words (fleder comes from flattern, "to flap"; spitz, or "point," refers to the shrew's pointy nose or rather head shape), it becomes possible to provide a clear name—or almost clear, at least, because there are many bat and shrew species, but more on that later.

Both names, of course, imply affiliation with mice, and that's the sticking point. In zoological terms, mice are a group of rodents known at the higher level of classification as Muroidea, "muroids" or the "mouse-like." The group includes quite the mix of animal groups, with occasionally curious names like zokor, blind mole-rat, spiny tree mouse, and Chinese pygmy dormouse, not to mention our pet hamsters and those domestic but unwelcome mice and rats. Common to all muroids are sundry and complex structural features in the skull, coupled of course with the oversized, continually growing incisors typical of rodents. Beyond that, although endless evolutionary gimmickry can revolve around this mouse theme (long or short legs, different fur colors and tail lengths, and much more), and even without biological expertise, most muroids tend to be identifiable as mice, if only vaguely.

Zoologically speaking, a mere mouse-like appearance is insufficient to denote a muroid. Instead, the specific anatomical features of the skull must be in evidence.

Field, house, and deer mice are familiar to many North Americans, although they typically live hidden away, and we don't often encounter them. These animals with the "mouse" base in their name are truly mice in the zoological sense.

The same cannot exactly be said for the bat and shrew—the fledermaus and spitzmaus—despite their names. Neither of them is even a rodent or, consequently, a muroid. Then what are they?

In the classification of mammals, a whole series of groupings is traditionally distinguished, usually assigned the rank of order within the class of mammals. Depending on scientific opinion, there are 25 to 30 of these orders of mammals. Rodents comprise one of these orders, to which muroids and several other groups of mammals belong.

Bats, meanwhile, are typical representatives of the order of flying mammals. Their scientific name is Chiroptera, from the Greek words chiros (hand) and pteros (wings). Chiroptera, then, means "hand-flier," which is a fitting name for bats and their closest relatives, flying foxes.

The systematic placement of the shrew, or spitzmaus, is determined in much the same way. They, too, fail to possess the mouse characteristics in question, although they do share traits with moles and hedgehogs, as well as with the solenodon (meaning "slotted tooth"), which is a venomous critter native exclusively to the Caribbean islands. They are now situated under the wondrous designation Eulipotyphla, but only since 1999. How they are related—along with ties to an array of other mammal families, such as tenrecs, desmans, and golden moles—has not been conclusively explained.

Experts have known for a long time—since Linnaeus's Systema Naturae at the latest—that neither bats nor shrews are related to mice, to which common parlance pays no heed. The fledermaus and spitzmaus comfortably maintain their spots in the lexicon.

 
 

One of the first mammal biologists to campaign for the standardization of German mammal names was Hermann Pohle. Born in Berlin in 1892, Pohle remained faithful to the city until his death and spent a large part of his life working at the natural history museum there. His career as a mammal biologist started early, when as a university student he worked as an unpaid hireling in the museum's famed mammal collection. Through diligence, endurance, and scientific acumen, he worked his way up to head curator of mammals. He thus held one of the most influential positions, of both national and international significance, in the field of systematic mammal research.

In 1926, Pohle—along with Ludwig Heck, the former director of the Berlin Zoo, and a number of other colleagues—founded the German Society for Mammalogy, of which he was the first head. Pohle thus had his finger on the pulse of mammal research, as it were, and he followed the history of the society over the next five decades "with keen interest," as one biographer noted.

In addition to his work as a researcher and curator of the mammal collection at Berlin's Museum für Naturkunde (Museum of Natural History), Pohle's interests also lay with German mammal names. Not only did he push for standardization of names, Pohle also campaigned to have existing names assessed for scientific plausibility and changed, should they not pass (his) zoological muster.

In 1942, Pohle published a summary article addressing the question, "How many species of mammals live in Germany?" He appended a comprehensive list of all German mammals, each with its correct "technical name," as Pohle called it, as well as its corresponding German name. When it came to the various species of spitzmaus (of which the Germans have eight, incidentally, despite the long-standing impression that there is "the" one and only shrew) and the 16 species of bats that have the base word "fledermaus" in their name, Pohle consistently uses alternative terms. The eight shrew species thus became waldspitzer, zwergspitzer, alpenspitzer, wasserspitzer, mittelspitzer, feldspitzer, gartenspitzer, and hausspitzer. For the bats, the base of their compound name was changed to fleder: teichfleder, langfußfleder, wasserfleder, and so on, all the way to a term of particular elegance, wimperfleder.

Pohle's article, which predates the society's 15th General Assembly and Hitler's emotional veto by more than a year, is a particularly interesting source because he also shares his actual motivations for the suggested changes. His emphatic objective is to see "the term 'Maus' disappear, responsible as it is for laypersons' wont to lump the animals together with actual mice."

In the estimation of these laypersons, mice are something "ugly and destructive that must be fought, or ideally exterminated." Shrews and bats, harmless as they are to humans, are thus subject to the same brutal fate. Pohle hopes for a "shift in perspective" to occur, once the endangered animals are no longer referred to as mice.

What to do, then? Pohle would prefer the term spitz for spitzmaus, but it's already been assigned to a dog breed. Rüssler could also work, only it already applies to some other insectivore. That leaves spitzer, a name that emphasizes the pointy head as a distinguishing characteristic and is still available.

Pohle wants a name for bats without "maus" but happily with a nod to the animals' flying ability. Most names of this kind are already employed for birds, and "flatterer" or "flutterer" could only logically be used for a certain population of bats, namely, those bad at flying. "Flieger" or "flyer," another hot candidate, is also in use by various other animal groups.

But why, Pohle asks the reader, would one even need to say "fledermaus," when "fleder" actually makes perfect sense? Pohle mentions that the original meaning of "fleder" was different, but few people were aware of this fact anymore.

On the off chance that he was correct in this assessment, let it be noted that fledermaus can be traced back to the 10th century, to the Old High German "vledern" or "flattern" (the infinitive form of "flatterer"). The image of the bat as a "fluttering mouse" has existed since this time in many languages, including "flittermouse" in English. A number of other German terms exist for bats. In some regions of Germany, such as Rhineland-Palatinate and Southern Hesse, the Old High German "fledarmus" is said to have been used to describe nocturnal creatures, such as moths. There, bats were apparently called "speckmaus," instead of fledermaus, because while hibernating, they could be seen hanging like pieces of bacon (speck) in the smoke.

Pohle's dedication to promoting the protection of bats and shrews through a bold name change reached its temporary culmination a year later, when—at the 15th General Assembly of the German Society for Mammalogy in Berlin—a resolution was passed on a universal and binding adoption of the spitzer- and fleder-based names Pohle had suggested. The results are known: Hitler was not amused.

 
 

We can only guess at what Hitler's actual motive was in issuing such drastic threats to prevent the name alterations proposed by the German Society for Mammalogy. It could have been his outrage that in 1942—hard times because of the war—leading German intellectuals were concerned with something so unimportant and banal as the appropriateness of animal names. Perhaps this anecdote is just a further example of Hitler's hostility toward intellectuals.

It is ultimately unclear, even, to what extent Hitler was the driving force behind this directive or whether this is a case of subordinates "working towards the Führer," as historian Ian Kershaw describes it. Conceivably, after reading the Berliner Morgenpost, Hitler may have remarked negatively regarding the zoologists' plans. His circle—in this case, Bormann—may have immediately interpreted this as "the Führer's will" and sprung to action accordingly. As for Pohle and his colleagues, it can't have mattered much whether the "invitation" to the Eastern Front came directly from Hitler or was communicated in an act of premature obedience.

Whatever the case may be, Pohle's suggested name changes did not fail because of Hitler's intervention, which presumably resonated as little with the German-speaking public as the original notice. Pohle failed because he wanted to take the basic idea of a standardized naming system out of the scientific context and transfer it into the realm of vernacular. Everyday German is not formally and officially regulated, and like every other vernacular, it follows different rules than scientific speech. It is shaped by a multitude of factors and influences that have their own unpredictable dynamic, which leads to some word usages changing while others stabilize.

In kindergarten, we learn that small, furry four-legged animals with a tail are "mice." This act of naming fulfills the exact function expected of it. It "tags" specific linguistic content—a meaning—that is generally understood. The difference between muroids and insectivores, which is important to zoologists, has no application in everyday confrontations with "mouse-like" animals and makes no difference to most people. A mouse is a mouse, whether a striped field mouse or a shrew.

10 Rad Gifts for Hikers

Greg Rosenke/Unsplash
Greg Rosenke/Unsplash

The popularity of bird-watching, camping, and hiking has skyrocketed this year. Whether your gift recipients are weekend warriors or seasoned dirtbags, they'll appreciate these tools and gear for getting most out of their hiking experience.

1. Stanley Nesting Two-Cup Cookset; $14

Amazon

Stanley’s compact and lightweight cookset includes a 20-ounce stainless steel pot with a locking handle, a vented lid, and two insulated 10-ounce tumblers. It’s the perfect size for brewing hot coffee, rehydrating soup, or boiling water while out on the trail with a buddy. And as some hardcore backpackers note in their Amazon reviews, your favorite hiker can take the tumblers out and stuff the pot with a camp stove, matches, and other necessities to make good use of space in their pack.

Buy it: Amazon

2. Osprey Sirrus and Stratos 24-Liter Hiking Packs; $140

Amazon

Osprey’s packs are designed with trail-tested details to maximize comfort and ease of use. The Sirrus pack (pictured) is sized for women, while the Stratos fits men’s proportions. Both include an internal sleeve for a hydration reservoir, exterior mesh and hipbelt pockets, an attachment for carrying trekking poles, and a built-in rain cover.

Buy them: Amazon, Amazon

3. Yeti Rambler 18-Ounce Bottle; $48

Amazon

Nothing beats ice-cold water after a summer hike or a sip of hot tea during a winter walk. The Yeti Rambler can serve up both: Beverages can stay hot or cold for hours thanks to its insulated construction, and its steel body (in a variety of colors) is basically indestructible. It will add weight to your hiker's pack, though—for a lighter-weight, non-insulated option, the tried-and-true Camelbak Chute water bottle is incredibly sturdy and leakproof.

Buy it: Amazon

4. Mappinners Greatest 100 Hikes of the National Parks Scratch-Off Poster; $30

Amazon

The perfect gift for park baggers in your life (or yourself), this 16-inch-by-20-inch poster features epic hikes like Angel’s Landing in Zion National Park and Half Dome in Yosemite National Park. Once the hike is complete, you can scratch off the gold foil to reveal an illustration of the park.

Buy it: Amazon

5. National Geographic Adventure Edition Road Atlas; $19

Amazon

Hikers can use this brand-new, updated road atlas to plan their next adventure. In addition to comprehensive maps of all 50 states, Puerto Rico, Canada, and Mexico, they'll get National Geographic’s top 100 outdoor destinations, useful details about the most popular national parks, and points on the maps noting off-the-beaten-path places to explore.  

Buy it: Amazon

6. Adventure Medical Kits Hiker First-Aid Kit; $25

Amazon

This handy 67-piece kit is stuffed with all the things you hope your hiker will never need in the wilderness. Not only does it contain supplies for pain, cuts and scrapes, burns, and blisters (every hiker’s nemesis!), the items are organized clearly in the bag to make it easy to find tweezers or an alcohol wipe in an emergency.

Buy it: Amazon

7. Hiker Hunger Ultralight Trekking Poles; $70

Amazon

Trekking poles will help increase your hiker's balance and stability and reduce strain on their lower body by distributing it to their arms and shoulders. This pair is made of carbon fiber, a super-strong and lightweight material. From the sweat-absorbing cork handles to the selection of pole tips for different terrain, these poles answer every need on the trail. 

Buy it: Amazon

8. Leatherman Signal Camping Multitool; $120

Amazon

What can’t this multitool do? This gadget contains 19 hiking-friendly tools in a 4.5-inch package, including pliers, screwdrivers, bottle opener, saw, knife, hammer, wire cutter, and even an emergency whistle.

Buy it: Amazon

9. RAVPower Power Bank; $24

Amazon

Don’t let your hiker get caught off the grid with a dead phone. They can charge RAVPower’s compact power bank before they head out on the trail, and then use it to quickly juice up a phone or tablet when the batteries get low. Its 3-inch-by-5-inch profile won’t take up much room in a pack or purse.

Buy it: Amazon

10. Pack of Four Indestructible Field Books; $14

Amazon

Neither rain, nor snow, nor hail will be a match for these waterproof, tearproof 3.5-inch-by-5.5-inch notebooks. Your hiker can stick one in their pocket along with a regular pen or pencil to record details of their hike or brainstorm their next viral Tweet.

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Meet Your Home's Microbes in The Great Indoors

Taylor Wilcox/Unsplash
Taylor Wilcox/Unsplash

This year, you’ve probably been spending more time than you ever expected at home. You might be sharing space with family members, roommates, pets—and an entire universe of microbes. In The Great Indoors: The Surprising Science of How Buildings Shape Our Behavior, Health, and Happiness, science journalist Emily Anthes investigates homes, offices, schools, hospitals, and other places where we live, work, and play. She looks at how the design of our surroundings affects major aspects of our lives, even when we don’t realize it. In this excerpt, she explores the thriving communities of bacteria and fungi with which we share our abodes—and what they reveal about us.

In 2010, microbiologist Noah Fierer made his first foray into the indoor microbial world, cataloging the bacteria present in 12 public restrooms at the University of Colorado Boulder, where he teaches. (Among the findings: The floor and the toilet handles were home to similar kinds of bacteria, suggesting that some bathroom-goers were flushing the toilet with their feet—“a practice well known to germaphobes and those who have had the misfortune of using restrooms that are less than sanitary,” Fierer and his colleagues reported.) The following year, he studied the microbes in residential kitchens and partnered with Rob Dunn to launch the Wild Life of Our Homes project. They began with a small pilot study in North Carolina, recruiting 40 families to run cotton swabs across seven surfaces inside their homes: a countertop, a cutting board, a refrigerator shelf, a pillowcase, a toilet seat, a TV screen, and the trim around an interior doorway.

The homes were crawling with microbial squatters—more than two thousand types, on average. Different locations within the homes formed distinct habitats: kitchens harbored bacteria associated with food, while doorways were covered in species that typically live in leaves and soil. From a microbiological perspective, toilet seats and pillowcases looked strikingly similar; both were dominated by bacteria that typically live on our skin and in our mouths.

Beyond these commonalities, there was a lot of variation among the homes, each of which had its own microbial profile, sheltering a slightly different collection of organisms. But the researchers couldn’t explain why. So Fierer and Dunn launched a second study, asking more than one thousand families living across the United States to swab the dust that had collected on the trim around their interior doorways.

“We focused on that because nobody ever cleans it,” Fierer told me. “Or we don’t clean it very often—maybe you’re an exception.” (I am not.) Because the dust collects over months or years, the duo hoped it would give them the broadest possible look at indoor life, an inventory of the organisms that had floated, crawled, and skittered through the homes over the previous months and years. As Dunn put it: “Each bit of dust is a microhistory of your life.”

Back in the lab, the team analyzed the DNA fragments present in each dust sample, listing every organism that made an appearance. The numbers were staggering. In total, the indoor dust contained DNA from more than 116,000 species of bacteria and 63,000 species of fungi. “The shocker was the diversity of fungi,” Dunn told me. There are fewer than 25,000 species of named fungi in all of North America, which means that our houses could be teeming with organisms that are essentially unknown to science. In fact, when the researchers compared the indoor dust to samples that the volunteers had taken from the trim around an exterior door, they found that there was more microbial diversity inside the homes than outside of them.

Scientific American/Farrar, Straus and Giroux

Some of the species that Fierer and Dunn identified originate outside, hitching rides into our homes on our clothes or drifting in through open windows. (And they may not all be alive by the time they turn up inside; DNA sequencing can identify the organisms that are present in a sample, but it can’t distinguish between living creatures and dead ones.) Other kinds of bacteria actually grow in our homes—in our walls and our pipes, our air conditioning units, and our dishwashers. Some sprout on our houseplants or our food.

And a lot of indoor microbes, it turns out, are living on us. “We’re constantly shedding bacteria from every orifice and body part,” Fierer said. “It’s nothing to be grossed out about. It’s just the way it is.” Our individual microbiomes—the collection of microorganisms that live in and on our bodies—are unique, and we each leave our own microbial signatures on the places we inhabit. In one innovative study, re- searchers tracked three families as they moved into new homes; each family’s distinct blend of microbes colonized its new residence within hours. The scientists—led by Jack Gilbert, a microbial ecologist then at the University of Chicago—could even detect the individual microbial contributions of each family member. “People who spent more time in the kitchen, their microbiome dominated that space,” Gilbert explained. “People who spent more time in the bedroom, their microbiome dominated there. You could start to forensically identify their movement.”

Indeed, the bacteria that turn up inside a home depend enormously on who lives there. Fierer and Dunn found that Lactobacil­lus bacteria, which are a major component of the vaginal microbiome, were most abundant in homes in which women outnumbered men. When men were in the majority, different bacteria thrived: Roseburia, which normally live in the gut, and Corynebacterium and Derma­bacter, which both populate the skin. Corynebacterium is known to occupy the armpit and contribute to body odor. “Maybe it means that men’s houses smell more like armpits,” Dunn ventured. “Microbially, that’s a fair assessment.” The findings may be due to sex differences in skin biology; men tend to have more Corynebacterium on their skin— and to shed more skin microbes into the environment—than women do. (The researchers also acknowledge the possibility that a bachelor pad’s bacterial profile could be the result of “hygiene practices.”) In a subsequent study, Fierer and his colleagues showed that they could accurately predict the sex of the students living in a college dorm room simply by analyzing the bacteria in its dust.

Meanwhile, dogs introduce their own drool and fecal microbes into a home and track soil dwellers in from outside. (Dog owners never seem too bothered when Dunn tells them that Fido is smuggling an entire microbial zoo into their homes. “It’s a pretty fine conversation most of the time,” he told me. On the other hand, he noted, “If I say that every time your neighbor comes over, that he brings over a mix of beneficial microbes and pathogens, it just makes people scrub.”) Cats change a home’s microbial makeup more modestly, perhaps because they are smaller and venture outside less often. Using the dust DNA alone, Fierer and Dunn were able to predict whether a home contained a dog or a cat with roughly 80 to 90 percent accuracy.

While the bacteria in our homes mostly comes from us (and our pets), the fungi are another story. Fungi are much less abundant in our own microbiomes, and our houses are dominated by fungal species that originate outdoors. A home’s fungal signature, Fierer and Dunn found, was largely determined by where it was located. Houses in eastern states had different fungal communities than those in western ones. Ditto homes in humid climates compared with those in dry ones. The geographic correlation was so strong that Fierer and Dunn could use fungal DNA to determine, to within about 150 miles, where a house dust sample originated.

Fierer and Dunn did identify more than 700 kinds of fungi that were more common indoors than out, including a variety of household molds, yeasts, edible mushrooms, and fungi that live on human skin. Homes with basements had different fungi than those without them. And because some species of fungi feed on wood and other building materials, what our homes are made of affects the fungi that live there. “It’s kind of a ‘three pigs’ thing,” Dunn told me. “A stone house feeds different fungi from a wood house from a mud house. Because unlike the bacteria, they’re eating the house.”

 

Some of the microbes that inhabit our homes are known to cause disease. Black mold, which grows in and on our walls, can trigger allergies and respiratory problems. Aspergillus fumigatus, a fungus that can cause lung infections in people with weakened immune systems, lives in our pillows. Legionella pneumophila, a bacterium that causes Legionnaires’ disease, loves indoor plumbing. It nestles inside hot water tanks, cooling towers, and faucets, and spreads through airborne, or aerosolized, droplets of water. Streptococcus bacteria—which can cause strep throat, sinus and ear infections, pinkeye, meningitis, and pneumonia—are more abundant inside our homes than outside them, Fierer and Dunn found. Though the mere presence of these microbes isn’t necessarily dangerous, and not all strains cause illness, buildings can provide an infrastructure that helps diseases spread. Airborne influenza can waft through an office building’s ventilation system; a spray of Strepto­coccus can turn a doorknob into a booby trap.

But many indoor microbes are completely innocuous, and some may even have lifelong health benefits. In recent decades, the rates of asthma, allergies, and autoimmune diseases have skyrocketed in industrialized nations. Some scientists have theorized that the increasing prevalence of these diseases may be the fault of our modern lifestyles, which keep us at a distance from the robust microbial menageries that surrounded our ancestors for most of human evolution. As a result, our immune systems never get properly trained.

Evidence has been accumulating to support this theory. Studies show that children who live with dogs, which increase the richness and diversity of bacteria in a home, are less sensitive to allergens and less likely to develop asthma. (A dog might be the immune system’s best friend.) Children who grow up on farms, and are exposed to livestock and their microbes, appear to be similarly protected from allergies and asthma.

Some of the most compelling evidence comes from research on two American farming communities: the Amish and the Hutterites. Although the groups have much in common—including large families and Central European ancestry—just 5 percent of Amish kids have asthma, compared to 21 percent of Hutterite children. The communities also have distinct farming customs. The Amish, who generally eschew electricity, live on single-family farms and employ traditional agricultural methods, using horses to plow their fields. It’s not uncommon for Amish children to play in the family barns, which are typically located near their homes. The Hutterites, on the other hand, live together on big, industrial farms, complete with high-tech tools and equipment, and their children have less contact with livestock.

These differences may affect the children’s microbial exposures and the development of their immune systems. In 2016, scientists reported that house dust collected from Amish households had higher levels of endotoxins—molecules contained in the cellular membranes of some bacteria—than dust from Hutterite homes. What’s more, when they drew blood from kids in both communities, they found that compared to Hutterite children, Amish children had more neutrophils, white blood cells that help the body fight infection, and fewer eosinophils, which play a critical role in allergic reactions.

The researchers also whipped up some house-dust cocktails, mixing dust samples from Amish and Hutterite homes with water, and then shooting the slurries into the nasal passages of young mice. Then they exposed the mice to allergens. The mice that had received the Hutterite dust responded as expected; their airways trembled and twitched. But the mice that had received the Amish dust continued to breathe relatively freely, seemingly protected from this allergic response.

Although there’s still a lot to learn, the science suggests that a healthy home is one that’s full of uninvited guests. “We are exposed to microbes every day, and a lot of these are harmless or potentially beneficial,” Fierer told me. “We don’t want a sterile house.” Which is good, because it turns out that I don’t have one.