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What's Living in You?

Discover how a world of microbes living in and on you can make you sick-and keep you healthy.

Six Microbes Living in You

<div class="rich-text">Microbes colonize our skin, guts, and bodies—but we wouldn’t be ourselves without them.</div>
<div class="rich-text">Microbes colonize our skin, guts, and bodies&mdash;but we wouldn&rsquo;t be ourselves without them.
</div>
<div class="rich-text"><h3>
 Six Microbes Living in You
</h3>

 Published May 9, 2018


 
 Onscreen:
 
 These six microbes may be living in you.


 1.
 
 Dermatobia hominis
 


 This parasite, also known as a botfly, grows on animals in the rainforests of central and south America. The female botfly glues her eggs to a mosquito. When the eggs detect the host’s body heat, they hatch and the larvae crawl into the host.


 They grow for about 3 months—until they’re the size of large peanuts. They then drop to the ground and pupate. After a month, the adults emerge to mate and repeat the cycle.


 2.
 
 Demodex brevis
 
 and
 
 Demodex folliculorum
 


 These parasites live on your face…and all over your body. At night, they come out of your pores and have sex on your face. Then they retreat into your pores to lay eggs. You can’t get rid of them. Not even showering or washing your face helps.


 3.
 
 Helicobacter pylori
 


 For years, doctors believed ulcers were caused by stress and unhealthy life choices. But in the 1980s, two Australian doctors examined biopsies of gastric ulcers. They discovered a new bacterium,
 
 H. pylori
 
 .


 To prove the cause of ulcers was the bacteria, one of the doctors used himself as a test subject. It worked. Before long, his stomach was teeming with
 
 H. pylori
 
 and he developed ulcers. Antibiotics killed off the bacteria, curing him.


 And in 2005, they won the Nobel Prize for this medical breakthrough. But today some researchers think that killing off all our
 
 H. pylori
 
 might contribute to other diseases


 4.
 
 Escherichia coli
 


 There are dozens of
 
 E. coli
 
 strains and they’re mostly harmless. Some help break down your last meal and produce vitamins your body needs. But some strains can cause illness, putting thousands of Americans in hospitals and killing about 30 people every year.


 5.
 
 Clostridium difficile
 


 This bacterium can cause colon inflammation and diarrhea. It lives in your gut naturally and it’s usually kept at bay by good bacteria. But
 
 C. diff
 
 can run rampant if you’re taking antibiotics that kill off these good bacteria.


 Every year, thousands of people can die from
 
 C. diff
 
 infections. In some cases, doctors may suggest a fecal transplant to restore your G.I. tract’s healthy gut bacteria. And it works ~90% of the time!


 6.
 
 Toxoplasma gondii
 


 This parasite infects the brains of mice. Healthy mice are scared of the smell of cat urine. But infected mice don’t run away, so they have a greater risk of being eaten. Which is what the parasite wants since it can only breed if it reaches the stomach of a cat.


 But you don’t have to eat mouse to get toxoplasmosis. You can get it from raw meat, unwashed vegetables, or from kitty litter. Which is why pregnant woman are advised to not handle kitty litter. The infection can cause miscarriage and premature birth.


 Viruses, fungi, and bacteria are all around us.They colonize our skin, guts, and bodies. We’re outnumbered. But without them, we wouldn’t be ourselves.
</div>

<div class="rich-text">Microbes colonize our skin, guts, and bodies—but we wouldn’t be ourselves without them.</div>

In Living Color

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 “I applied streaks and blobs of colors onto the canvas with a palette knife and I made them sing with all the intensity I could…” Wassily Kandinsky
 

 Who can paint pictures that sing? A synesthete, that’s who. In Kandinsky’s case, his colors and painted strokes set off sounds or musical notes. And sometimes the other way around, too.
 
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/kandinsky.comp-7_t1U8tyv.width-990_QpuNoZb.jpg"/>
<figcaption>Composition VII, No. 196, 1913. What do you hear?
 
 </figcaption>
</figure>

 Kandinsky was born in Moscow in 1866, but grew up in Odessa playing the piano and cello at a young age. He studied law and economics and lectured at the Moscow Faculty of Law.
 

 At the age of thirty, he abandoned law and moved to Munich for art school. He studied life-drawing, sketching and anatomy, the traditional subjects for artists at the time, after not recognizing a Monet haystack. Kandinsky’s early style was marked by a liberation of color from defined form. When his teacher, Franz Stuck, told him that his palate was too bright, Kandinsky worked in black-and-white for a year, focusing more on form and less on the music of his colors.
 

 What effect did that absence of color have on his condition and his art? Returning to Russia, he worked more on the analysis of color and form. He and his wife emigrated to Paris in 1926-1933, where he painted in oil and water colors. Sadly, the Nazis considered his work degenerate and destroyed many of them.
 

 When Steffie Tomson mentioned Kandinsky, I instantly remembered studying his paintings in my college art history course years ago. Vivid colors and abstract lines mark his art. When I looked at his paintings again, after watching Steffie, I was struck by the intensity of his work.
 

 But it all makes sense now. It seems as if he feels his strokes and I can almost sense now the music he heard as he applied the paint to the canvas.
 

 I’ll have a new respect for his art next time I see it up close. I can’t wait to get to the museum. What a way to view the world—in living color.
 
</section>

You Have Mites Living on Your Face

The Star In You

<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/star-in-you-01.width-990_bMGTnLN.jpg"/>
<figcaption>As hard as it might be to believe, every atom in your body, astrophysicists say, originated billions of years ago in a star or in the explosive aftermath of the Big Bang. Here, a close-up of Polaris, the North Star.
 
 <div class="credit">NASA, ESA, G. Bacon (STScI)</div>
</figcaption>
</figure>


	It's true, according to astrophysicists. You and everything around you, every single natural and man-made thing you can see, every rock, tree, butterfly, and building, comprises atoms that originally arose during the Big Bang or, for all but the lightest two or three elements, from millions of burning and exploding stars far back in the history of the universe. You live because stars died; it's that simple.

	How is this so? How can you possibly be a walking galaxy of fossil stardust? Well, the story is not a new one, but it bears retelling, if only because its working out was one of the finest achievements of 20th-century astrophysics—and because it's so astonishing.
<h3>
	The start of it all</h3>

	The story begins at the beginning, as in the Big Bang. That is when, astrophysicists say, all the hydrogen in the universe came into being. Initially it was just protons, and then, as the young universe expanded and cooled, these became bound to electrons, forming hydrogen atoms. The very hydrogen atoms in the H2O that makes up over half your body were born then. They didn't come from your parents; they came from the early universe. Did you have any idea you have atoms in your body that are over 13 billion years old?

	If you could separate one hydrogen atom from one molecule of water in your body, shrink down to its atomically tiny size like the scientists in <cite>Fantastic Voyage</cite>, then reverse time and follow it back to through its unimaginable lifetime, you would find yourself in the immediate aftermath of the Big Bang. That very hydrogen atom, an atom now inside you as you read this, has remained unchanged since the beginning of time.

<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/star-in-you-02.width-990_xUiR7am.jpg"/>
<figcaption>Over 13 billion years since the Big Bang, hydrogen and helium still make up most of the visible matter in the universe. Nearly 10,000 galaxies appear in this Hubble Ultra Deep Field image.
 
 <div class="credit">NASA, ESA, and N. Pirzkal (STScI/ESA)</div>
</figcaption>
</figure>


	The Big Bang also churned out helium, the next lightest element. You don't have any helium in you, unless you just sucked the gas out of a birthday balloon. But helium is the second most common element after hydrogen. Together they make up more than 98 percent of the matter in the universe. (Luminous matter, that is; dark matter is a whole other story.) A smattering of lithium (element 3) and one or two other of the lightest elements also formed in the Bang, but these were negligible.

	Everything else, every other chemical element, including carbon, oxygen, nitrogen, and all the other elements essential for your life, is thought to have been fabricated in stars.

	How? Well, the story is either simple or horrendously complex depending on whether you're a science writer or a scientist. Here's the simple story:
<h3>
	Table for 118</h3>

	First, what are we talking about when we talk about an element? A chemical element is a substance that cannot be broken down or changed into another substance using chemical means. It can be changed using nuclear means, which is what happens inside stars.
<blockquote>
	Every second, the sun converts about 500 million tons of hydrogen into helium.</blockquote>

	As we learn in high school chemistry—and can remind ourselves with a quick glance at the <a href="http://commons.wikimedia.org/wiki/Periodic_Table_of_Elements" target="_blank">Periodic Table</a>—hydrogen, the lightest element, has one proton in its nucleus and thus is given the atomic number 1. Helium has two protons and so is number 2, and so on all the way up to uranium, which, with 92 protons in its nucleus, is the heaviest of the "naturally occurring" elements.

	Remarkably, all life on Earth, all everything we see around us, consists of various combinations of those 92 elements. There are still heavier elements, ranging from neptunium (93) all the way up to the unofficially named ununoctium (118), though with the exception of trace amounts of neptunium and plutonium (94), these are not found naturally on Earth.

<figure class="richtext-image full-width">
<img alt="early star formation" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/star-in-you-03.width-990_ZiiRmmD.jpg"/>
<figcaption>An artist's impression of how the very early universe—less than one billion years old—might have looked during an intense period of hydrogen conversion into myriad stars
 
 <div class="credit">Science: NASA and K. Lanzetta (SUNY). Art: Adolf Schaller for STScI.</div>
</figcaption>
</figure>

<h3>
	Stars are born</h3>

	How did—and do, for the process continues today—all the chemical elements first come into existence?

	Several hundred million years after the Big Bang, about 13 billion years ago, the hydrogen and helium in the early universe began coalescing into gas clouds, which, in turn, collapsed into the first stars. Gravity, that not-to-be-denied force, caused these newborn stars to contract, heating their cores to temperatures high enough to ignite their hydrogen and trigger its fusion into helium.

	This is the first link in a chain of thermonuclear reactions that, depending on the size of the star and its fate, bring about the genesis of all the other chemical elements up to about californium, element 98. (Heavier elements than that are produced only in particle accelerators, physicists believe.) Imagine starting out in your kitchen with just a single natural ingredient and, after baking it in your oven, winding up with all other possible natural ingredients. This is what the universe has done with hydrogen.

	The burning of H to He is what our star, the sun, does for a living. In the searing heat of its core—about 27 million °F—the reaction of four hydrogen nuclei fusing to become one helium nucleus happens over and over and over again, ad infinitum. Every second, the sun converts about 500 million tons of hydrogen into helium. (And for every helium atom formed, roughly a trillion photons are emitted from the sun's surface. This is why we wear sunglasses.)
<h3>
	Cooking elements</h3>

	Our star enables us to live, but at this stage in its own life, it doesn't give us any elements heavier than helium. It's not massive enough. With stars more massive than ours, and up to about eight times its mass<a class="asterisk-note" href="#footnote1" title="See footnote">*</a>, gravity is forcible enough to compress the core sufficiently to trigger nuclear reactions that produce heavier elements, starting with carbon (element 6) and oxygen (8). In such cores, the heat is high enough, about 180 million °F, to force three helium nuclei to fuse into a carbon nucleus, or four helium nuclei into an oxygen nucleus, millions of times over. This will happen in the sun when it becomes a red giant in five billion years.

<figure class="richtext-image full-width">
<img alt="the sun" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/star-in-you-04.width-990_NtA6V8L.jpg"/>
<figcaption>In its fiery core, our star, the sun, produces only a single chemical element—helium—over and over again.
 
 <div class="credit">NASA</div>
</figcaption>
</figure>


	In very massive stars, those of more than eight solar masses, the force of gravity drives the temperature in the core up so outlandishly high that it triggers thermonuclear reactions that create elements all the way up to iron (26). At 1,080 million °F, carbon fuses into neon; at 2,700 million °F, oxygen fuses into silicon; and at 7,200 million °F, silicon fuses into iron.

	Iron, alas, marks a major turning point when it comes to fusing ever-heavier elements inside stars. All the way up to iron, every time a new fusion reaction occurs, some heat is released. With iron, no other rearrangement of nuclei can generate any more energy. But stars do form elements heavier than iron, including cherished ones like silver and gold, dangerous ones like radon and uranium, and ones you've never heard of (or could pronounce if you had) like praseodymium and ytterbium.
<h3>
	Two ways to you</h3>

	Stars have one of two ways to produce these heavier-than-iron elements—and, not incidentally, to get them and all the other elements forged in their nuclear furnaces out into space so they can be incorporated into new stars, planets, and people.
<blockquote>
	Some of that widely dispersed stardust is holding you up right now.</blockquote>

	The first way occurs in red giants. These are stars that have burned up all the hydrogen in their centers. When that happens, the star becomes, as the astrophysicist Craig Wheeler has put it, somewhat schizophrenic: The core loses energy, contracts, and heats up even as the envelope—the rest of the star outside the core—gains energy, expands, and cools (and appears redder). The expansion is quite, well, expansive: When our sun becomes a red giant, it will grow so large that it will engulf and evaporate the inner planets, including the Earth.

	Some red giants last long enough to create elements in their cores heavier than iron through something called the s-process, for slow. Over a time scale of thousands of years, the s-process can result in the manufacture of elements all the way up to bismuth (83). These get pulled to the star's surface by convection and sloughed off into space via the star's stellar wind. Some of that widely dispersed stardust is holding you up right now.

<figure class="richtext-image full-width">
<img alt="Cas-A supernova remnant" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/star-in-you-05.width-990_hXbgWiB.jpg"/>
<figcaption>This spectacular false-color image shows Cassiopeia A, the remnant of a supernova. At the center of the image lies the dead star, while surrounding it is the rapidly expanding shell of material blasted away from the star as it died.
 
 <div class="credit">NASA/JPL-Caltech/O. Krause (Steward Observatory)</div>
</figcaption>
</figure>

<h3>
	A real blast</h3>

	Elements heavier than bismuth only arise through the r-process, for rapid. How rapid? Seconds flat. The r-process is what happens when a star explodes in a supernova. It's easy for us to think of stars as lasting essentially forever, but the most massive stars survive only a few million years—a cosmic moment, really—and when they go, they go fast.

	What happens? When a red giant gets to the stage of having fused all its lighter elements and is left with an iron core, the star can no longer retain its equilibrium—heat energy pushing out as gravity pulls in. Gravity suddenly gains the upper hand, collapsing the core all at once to billions of times the density of the Earth. The star then blows itself apart in an astronomical cataclysm. For a brief period, it shines as brightly as an entire galaxy and releases as much energy as our sun will in its 10-billion-year lifetime.

	In the first few seconds, protons in the atoms created during the star's life collide with highly energetic neutrons, fashioning in an instant all the naturally occurring elements heavier than bismuth up to uranium, and even a few short-lived still-heavier elements such as plutonium and californium. All these blast out into space at millions of miles an hour, seeding the interstellar medium with the atoms that eventually end up in new stars, new solar systems, and, in your case, you.

<figure class="richtext-image full-width">
<img alt="Carina nebula" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/star-in-you-06.width-990_Ib37YPs.jpg"/>
<figcaption>In this view of the Carina Nebula, the Hubble Space Telescope captured a tumult of star birth and death. In the image, green corresponds to hydrogen, blue to oxygen, and red to sulfur—three of the 92 naturally occurring elements that space has bequeathed to us.
 
 <div class="credit">For Hubble Image: NASA, ESA, N. Smith (University of California, Berkeley), and The Hubble Heritage Team (STScI/AURA). For CTIO Image: N. Smith (University of California, Berkeley) and NOAO/AURA/NSF</div>
</figcaption>
</figure>

<h3>
	The birth of you</h3>

	Over time, molecular clouds of gas and dust out in deep space develop from those strewn elements and begin to contract under their own gravity. Such clouds are almost all hydrogen and helium, but they've got a scatter of heavier elements, too. And the most abundant elements begin to assemble into molecules, simple ones like water (H2O) and more complex ones like the sugar glycoaldehyde (C2H4O2). Astronomers can identify these compounds, and individual elements, using spectrometers.

	Eventually, a kind of raw-clay star called a proto-star forms, with a disk of material surrounding it that will eventually beget planets. That process happened in our own solar system about five billion years ago, resulting in the sun, the planets, and, five billion years later, you.
<blockquote>
	Did you have any idea you have atoms in your body that are over 13 billion years old?</blockquote>

	Just how those atoms and molecules that ended up on our planet went from non-living to living remains one of the great unanswered questions in science. But where the elements came from to start with has now been worked out, in broad strokes anyway, to astrophysicists' widespread satisfaction. It is an amazing story, isn't it?

	 

*While still being sorted out, current estimates hold that, in time, stars below about eight solar masses collapse into white dwarfs, a very dense ex-star; those between about eight and 30 solar masses explode in supernovae (and release their heavy elements); and, with certain exceptions, those between about 30 and 100 solar masses collapse into black holes (and retain their heavy elements).

Just what do astronomers mean when they say we're all made of star stuff?

Are We Living in a Computer Simulation?

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 What if your thoughts and everything you perceive are nothing but bits in a computer simulation designed to satisfy the curiosity of scientists with capabilities far beyond anything known to human beings?
 

 Maybe you’re thinking, “I already saw that movie.” Or, “What’s the point in speculating on some abstract philosophical theory that we can never test anyway?” Or maybe you just think it all sounds pretty far-fetched.
 

 But some philosophers are taking this idea, called the “simulation argument,” very seriously. Physicists have gone even further, suggesting that we might even be able to detect evidence that confirms it, if we know where to look.
 
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/matrix_620.width-990_Y20CnbD.jpg"/>
<figcaption>
</figcaption>
</figure>

 In 2003, philosopher Nick Bostrom of the University of Oxford made the first rigorous exploration of the simulation argument. The simulations he considered are different from those in movies like “The Matrix,” in which the world is simulated but the conscious minds are not—that is, where biological human beings with human brains interface with the simulated world. In Bostrom’s simulations, human consciousness is just another figment of the simulation.
 

 Bostrom assumes that the human mind is
 
 substrate-independent
 
 : that human consciousness isn’t strictly dependent on the biological brain itself, and that if we could physically replicate that brain in sufficient detail in another form (such as within a computer) it would also have the subjective experience of consciousness. The replication doesn’t have to be perfect. It just has to be good enough that the replicated being has a human-like subjective experience (a “mind”). An advanced civilization with sufficient computing power to pull this off would be classified as “posthuman.”
 

 What is the probability, then, that we ourselves are simulated minds?
 

 To calculate the probability that a randomly-selected human-like mind—let’s call it “you”—is a simulation, you would divide the number of simulated minds by the total number of all human-like minds (both simulated and non-simulated, or ”real”).
 
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/eq1.width-990_33xfIWe.jpg"/>
<figcaption>
</figcaption>
</figure>

 It might seem that there’s no way to make sense of these quantities, but keep in mind that the “simulated minds” in this case come from a posthuman civilization running detailed simulations of its own past. The total number of “simulated minds” will be a multiple of the “real minds” of the humans that existed before they reached posthuman status. This multiple will be the average number of simulations run by the society (although this argument doesn’t rule out multiple human-like societies existing, if you’re a fan of the somewhat-similar Drake equation). So if you divide both the numerator and denominator by the number of “real minds” (even though we have no idea what that number is), you reach the following:
 
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/eq2.width-990_Kl0WYr6.jpg"/>
<figcaption>
</figcaption>
</figure>

 Now we get to play with the numbers to see what happens. If the total number of simulations is very small, the ratio is very small. But if the total number of simulations is very large, the ratio will be close to one. Arguing from some reasonable assumptions about what drives the number of simulations, Bostrom explains that we can expect at least one of three scenarios:
 

 1. The fraction of civilizations that survive to the posthuman level is very small.
 
 2. The fraction of posthuman civilizations interested in running simulations is very small.
 
 3. The probability that you are a simulated mind is a simulation is very high.
 

 If options 1 and 2 are correct, we can relax: we’re probably real. But if you think that many civilizations do survive to become “posthuman,” and that many of those posthuman civilizations are indeed interested in running simulations, then option 3—that you are just a computer simulation—becomes a serious probability. It’s hard to tell, of course, as we have no direct experience with posthuman civilizations and their preferences, and there are a number of philosophical objections worthy of debate: Why would any posthuman civilization want to do this? Are Bostrom’s assumptions actually reasonable, or do they miss something vital? Is substrate-independence true, or is it impossible to replicate a human mind?
 

 Can physics offer any insight here? English cosmologist John D. Barrow addressed this question in a 2007 essay published in the book “Universe or Multiverse?,” in which he argued that the simulations might have limits. Even if posthuman simulators “have a very advanced knowledge of the laws of Nature, it’s likely they would still have an incomplete knowledge of them,” wrote Barrow. Any flaws or gaps in this knowledge “would of course be subtle and far from obvious, otherwise our ‘advanced’ civilization wouldn’t be too advanced.”
 

 If these gaps exist, as Barrow reasons, the result would be either glitches in the working of reality, or update “patches” to fix a glitch before it causes a problem. (Recall that in “The Matrix,” local changes to the Matrix caused déjà vu.) These patches could result in changes, over time, to the laws of nature. Barrow concludes:
 
<blockquote>

 […] if we live in a simulated reality we should expect occasional sudden glitches, small drifts in the supposed constants and laws of Nature over time, and a dawning realization that the flaws of Nature are as important as the laws of Nature for our understanding of true reality.
 
</blockquote>

 An un-peer-reviewed 2012 attempt at a more rigorous physical analysis of the situation, “Constraints on the Universe as a Numerical Simulation” by physicists Silas R. Beane, Zohreh Davoudi, and Martin J. Savage, reached the conclusion that “in principle there always remains the possibility for the simulated to discover the simulators.” Their specific prediction was that there might be limitations on cosmic ray energy levels if we live in a simulation. However, the above statement might be overstating their own case: rather than discovering the simulators, if we find that cosmic rays violate these limitations, we will have instead disproved that we’re in a simulation!
 

 This paper also predicts why our posthuman descendants might want to simulate our universe: to test out string theory. Currently, string theory is overpowered by a vast landscape of possible versions of string theory, and scientists haven’t figured out which one might describe our universe. Detailed simulations would allow posthuman races to test hypotheses about these universes, ruling out possible versions of string theory to zero in on the one that describes their real universe.
 

 If this is the case, Barrow must be right that even the simulators are working from incomplete knowledge. A civilization that is simulating the universe to explore the string theory landscape must not know everything about the laws of physics, and therefore we can reasonably expect gaps and flaws in the simulation.
 

 Indeed, these ideas suggest that our entire known universe is itself only a small part of a grand experiment to understand the most fundamental mysteries of the universe. And for the scientifically-minded among us, many may find that a worthy purpose to our simulated creation.
 


 Go Deeper
 
 

 Editor’s picks for further reading
 


<a href="http://www.simulation-argument.com/">
 Are You Living In a Computer Simulation? The Simulation Argument
 </a>
 
 Find an extensive list of articles, videos, and other media discussing the “simulation debate” at this site curated by Nick Bostrom.
 

 Backreaction:
 <a href="http://backreaction.blogspot.ca/2013/02/the-simulation-hypothesis-and-other.html">
 The simulation hypothesis and other things I don’t believe
 </a>
 
 Sabine Hossenfelder explains why “the simulation hypothesis is not among the things that keep me up at night.”
 
</section>

Some physicists and philosophers say your thoughts are just bits in a computer simulation.

You Might Have More Than 200 Types of Bugs Living in Your Home

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 If you think you live alone, think again. Your humble abode is also home to potentially hundreds of tiny arthropods. These unseen roommates consist of ants, beetles, book lice, and more.
 

 A team of scientists led by Matt Bertone of North Carolina State University surveyed visited 50 houses in and around Raleigh, North Carolina. From those, they were able to collect over 10,000 specimens—more than 500 morphospecies (or species that can be differentiated by their structure alone)—from large spiders to tiny creatures only a few millimeters long. Overall, the critters represented over 300 families over arthropods.
 

 Even more surprising, the scientists classified these insects and discovered that each home contained somewhere between 32 and 211 arthropod species. The study was
 <a href="http://news.nationalgeographic.com/2016/01/160119-insects-spiders-bugs-homes-science-animals/">
 published
 </a>
 Tuesday in the journal Peer J.
 
<figure class="richtext-image full-width">
<img alt="" class="richtext-image full-width" src="https://www.pbs.org/wgbh/nova/media/images/book-lice_2048x1152.width-990_KNIckNN.jpg"/>
<figcaption>This bug may look terrifying, but really it’s just an innocuous book louse.
 
 </figcaption>
</figure>

 Out of more than 550 rooms sampled, only five were completely bug-free.
 

 Here’s Mary Bates, writing for National Geographic:
 
<blockquote>

 There are probably even more bugs in our homes, since the researchers only collected arthropods from visible surfaces, excluding areas such as behind walls, under heavy furniture, and in drawers and cabinets, Bertone says.
 

 But he emphasizes that we shouldn’t be afraid of the bugs in our midst—the vast majority are not harmful. Typical household pests, such as German cockroaches, termites, and fleas, were only present in a few homes, the survey found. And those ubiquitous spiders found in every home are very benign—not to mention they eat pests.
 
</blockquote>

 The study, which is the first comprehensive survey of in-home arthropods, is just one of many other investigations into the complex built ecosystem; scientists are exploring the microbial inhabitants of
 <a href="http://to.pbs.org/1OJozIo">
 our houses
 </a>
 and
 <a href="http://to.pbs.org/1taxBWE">
 our hospitals
 </a>
 , as well.
 
</section>

 Photo credit:
 <a>
</a>
 Macroscopic Solutions