"The average house has thousands of species."

Dunn started out studying microorganisms and insects in rain forests, but his focus gradually shifted toward backyards and houses. "I eventually found myself in homes with the realization that a lot of what I'd done in jungles ... we could do under the bed and showers," he says. "And we were making the same kinds of discoveries I'd make in Bolivia or Ghana or Australia or anywhere else."

Dunn's new book, Never Home Alone, describes the tiny life forms he's found living in different parts of the home, including on floors and water faucets and in basements and heating, ventilation and air conditioning systems.

Dunn warns that, too often, people attempt to scrub away all microbes — without considering that some of the organisms may actually be beneficial. Take antibacterial soaps, for instance. Dunn notes that although antibacterial soaps kill pathogens, they also tend to favor some bacteria that are harmful to humans.

"They're really a great example of where we've gone too far in trying to kill everything around us, and it's had unintended consequences," he says.

Instead, Dunn suggests that humans would do better to accept the microbes that share our space.

"Our bodies don't exist but for the species that live on and in them," he says. "We can't scrub ourselves free of the rest of life. I hope to be never alone in that sense — to be really isolated from the rest of life would be a very sad thing."


Interview Highlights

On bacteria that live in showerheads

If you unscrew an American-style showerhead — where you have that pipe going into the showerhead and you can just unscrew the tip and then look into the showerhead — what you'll see is a kind of gunk on the inside of the showerhead. That's a biofilm, which is kind of an apartment that the bacteria collectively excrete and then live in. And it keeps them buffered from the flow of water, and so they make that little apartment and they live there. They eat essentially the nutrients that are flowing in the water, through the tap toward you. So when you shower, that's right above you, and a little bit of that breaks off each time you're showering and sort of sprays down onto you. Mostly that's harmless, but there's a group of bacteria called nontuberculous mycobacteria, which are relatives of leprosy and tuberculosis, and those sometimes establish in those biofilms....

People who are on municipal water supply — cities like Philadelphia and New York — that are heavily chlorinated, that chlorine seems to kill most of the bacteria in the water system. But it leaves those nontuberculous mycobacteria alone because they're chlorine-tolerant. So it kind of creates a world in which they can live happily without much competition. On the other hand, in well water (from rural environments or municipal water from some of the European cities that don't treat their water with chlorine), the mycobacterium seems very, very rare. And so we're accidentally making them common by trying to kill everything.

On a recent study on how black mold fungus gets into houses

One place that fungi actually start to pose problems is in your walls. It's been known for a while that if your house gets wet there's suddenly a bunch of kinds of fungi that show up kind of miraculously out of nowhere and grow super quickly. And one of those is this toxic black mold that people worry a lot about. But, interestingly, although lots of money is made on doing abatement of that mold, nobody had really figured out what was going on with this basic biology — until a few years ago when the scientist Birgitte Andersen started to try to figure out, 'When that mold shows up in houses, where has it come from?' ...

So what [Andersen] did was to look at brand new drywall right from the factory to figure out, 'In that drywall, are there already some fungi?' And she was able to show that that toxic black mold — along with several other fungi — basically come preloaded in your drywall. And then they wait in your drywall for months, or years, or however long it takes for the drywall to get wet. And then, once the drywall gets wet, they can eat your house from bottom to top with great joy and a little bit of laughter, I think. And so that's when the problem emerges, and nobody really had any understanding of that until a year and a half ago.

On bacteria that live in our armpits

I find armpit bacteria fascinating. We actually have these glands in our armpits that are not really sweat glands. Their only job is to feed armpit bacteria. So, you have very special microbes in your armpit that, historically, must have had a pretty important role that we still don't really understand. Because those glands are feeding those bacteria so much, we shed lots of those microbes around the house.

I actually think it's pretty likely that those are some of the microbes that are most important in defending us against pathogens. If you think about your body, it's not actually just covered in skin — the outside layer of your body is totally microbial. If a flu virus lands on your hand, the first thing it encounters is not your immune system; it's the layer of your microbes on your hand. I think that some of what those glands are doing may be to feed some of the microbes that best defend us from some of those things we actually have to worry about.

On the microbes cats and dogs bring into our homes

Cats and dogs (and especially dogs) have a huge impact on what's in your home. ... They affect the microbes everywhere in the house. Their microbes drift up and around all over the place. We can actually identify whether or not there's a dog in the house just based on the microbes — 99 percent of the time. And cats have an effect, too, but it's more subtle. ...

It's hard to know which effects the dogs and cats and their microbes are having, and they can be very complex. But one of the things we see is that people, especially in urban environments with dogs, tend to have kids who are less prone to allergy and asthma. One of the things that we think may be happening is that the dogs are actually a vehicle for the connection of those kids to just a little bit of nature. In these environments in which we've isolated ourselves so much from the rest of the world ... the dirt on our dog's paws may be enough connection to forestall allergy and asthma at least a little bit. We don't see that effect with cats, but with dogs it's pretty strong. By the same token, dogs in rural environments seem to have less effect on allergy and asthma. We think that's because in rural environments you're getting so many other exposures to nature that the dog matters a lot less.

On whether one day there might be a probiotic-type product to bring good microbes into our homes

It's not there now, but I look forward to a time when we can essentially garden our homes — fill them with species that benefit us and push out the species that don't. The closest I think we come to that is in the context of making fermented foods. We study sourdough bread a lot. When you make a sourdough starter, that starter has a bunch of microbes that produce lactic acid, and that lactic acid is key to the starter, but it also kills off any other bacteria. And so it's kind of a garden that weeds itself. And for me, that's a metaphor for what we'd like to have in our homes.

Sam Briger and Thea Chaloner produced and edited the audio of this interview. Bridget Bentz, Molly Seavy-Nesper and Deborah Franklin adapted it for the Web.

Copyright 2018 Fresh Air. To see more, visit Fresh Air.

TERRY GROSS, HOST:

This is FRESH AIR. I'm Terry Gross. You may be shocked to learn what's living in your home. The bacteria, the fungi, viruses, parasites and insects - probably many more than you imagined. But if that makes you think you'd better start scrubbing right away, my guest Rob Dunn says the real goal is to create a healthy balance of all the microbes that co-habit with us. You might be upsetting that balance with some antimicrobial sanitizers.

Dunn inventories microorganisms and insects in our homes the way other researchers might inventory the wildlife in rainforests. He and the team of scientists he works with analyze the populations of microorganisms living on floors, in basements, on water faucets, the surfaces of TVs, in HVAC systems, et cetera. They often work with citizen scientists around the country who swab parts of their home for analysis.

Dunn is the author of the new book "Never Home Alone: From Microbe To Millipedes, Camel Crickets And Honey Bees, The Natural History Of Where We Live." He's a professor in the Department of Applied Ecology at North Carolina State University and in the Natural History Museum of Denmark at the University of Copenhagen.

Rob Dunn, welcome to FRESH AIR. So I was prepared to learn that there would be bacteria and fungi in my home. I was prepared to learn that there was a lot of microwildlife. I was unprepared to learn how much and how really creepy some of it was going to be.

ROB DUNN: (Laughter).

GROSS: So let's start with just, like, an overview of, like, what's going on in our homes.

DUNN: So every surface, every bit of air, every bit of water in your home is alive. And every house, every building we've ever studied - all of those things are alive. And so the only choice you really get is which life. And so when we study homes, different homes have different kinds of life in those different places. And so you look in a hot water heater, and there are microbes that are - they've evolved to be able to deal with really, really high temperatures. If you look in your salt shaker, there are bacteria like bacteria you find in salt flats in the desert. If you look in your attic, there are interesting bugs in your attic that we don't see elsewhere. In your basement, you see cave animals.

And so your house has habitats. Each of those habitats is a little bit different. And there are collectively, across houses, hundreds of thousands of species. But the average house has thousands of species. And if you breathe in deeply right now, Terry, just inhale, in that breath - what's in your lungs at this very moment are thousands of species. And a couple of them are bad news, some of them you depend on, and most of them nobody's ever, ever studied in any real way.

GROSS: Why are you studying them?

DUNN: So I started off studying rainforests. I was fascinated by the fact that you could turn over a leaf, and every leaf seemed to have something new. And slowly my career drifted toward backyards. And then I eventually found myself in homes with the realization that a lot of what I'd done in jungles we could do under the bed and in showers. And we were making the same kinds of discoveries I'd make in Bolivia or Ghana or Australia or anywhere else. And so it was this realization that there were these discoveries people had overlooked right where I live, right where we all live, and that we can engage the public in helping us to make those discoveries. And once we figured that out, it was hard to go back.

GROSS: One of the things you've learned in studying the microecosystem of homes is that there are microbes that live in the extreme environments outdoors - in very cold temperatures or very hot temperatures - that could live indoors because our homes reproduce the extremes of the outdoors through things like the freezer and the boiler. So can you elaborate on that a little bit?

DUNN: Yeah. So if you look at a traditional home from a couple of hundred years ago, it would have been relatively open to the outdoor environment, and the conditions in it would have been very much like the conditions outdoors - a little bit buffered, but very similar. But in our modern, complex homes, we've actually - we've built a whole bunch of little structures in the home that replicate really extreme environments from elsewhere on Earth. You know, there - our freezers are kind of like the Arctic. Our ovens are like really, really hot deserts. And so when we do that, we're creating habitat for species that like those conditions.

GROSS: Last year, you were in Iceland filming a documentary near a bubbling, hot, sulfurous geyser. People studying the microbes of hot water taps and boilers in homes found similar microbes to the ones in this sulfurous geyser. So what are they, and what are they doing in our homes?

DUNN: Yeah, so these are really amazing microbes that - nobody saw them for many, many years even though people looked in places like geysers because when we study microbes, we often - we bring them into the lab, and we try to grow them in conditions that we think are nice. And so it's more or less like Florida temperatures. But these microbes actually can't grow at those temperatures. The - their environment has to be really hot, close to boiling water temperature, to grow.

And so it was only when people started to try to grow these microbes at really, really hot temperatures that they appeared. And so they were first discovered in geysers like the one I was near in Iceland. But later, the scientist who found them in Iceland and Yellowstone decided to look around his lab to see if in other hot conditions he might find them. And in the lab hot water tap, he was actually be able to find some of the same microbes that are found in those geysers.

And he would go on to show that if you go to laundromats, that the hot water heaters in laundromats are full of these microbes. And then if you - it turns out, if you go to houses, in houses, hot water heaters are full of them, too. And they live in that superhot water that we think is sort of sterilizing our water, and they love it. And they pour out of your hot water tap when you pour some hot water. They pour on to you when you take a shower. And they're fascinating little bugs.

GROSS: Are they harmful to us?

DUNN: They're not harmful in any way.

GROSS: Another study that's been done is on the biofilm of showerheads. I mean, while we're talking about some of the things that happen in the bathroom - so tell us about what's going on on showerheads.

DUNN: Yeah, so if you unscrew an American-style showerhead where you have the pipe going into the showerhead and you can just unscrew the tip and then look into the showerhead, what you'll see is a kind of gunk on the inside of the showerhead. And that's a biofilm, which is kind of an apartment that the - that bacteria collectively excrete and then live in. And it keeps them buffered from the flow of water. And so they make that little apartment, and they live there. And they eat essentially the nutrients that are flowing in the water through the tap toward you.

And so when you shower, that's right above you. And a little bit of that breaks off each time you're showering and sort of sprays down on you. And mostly that's harmless, but there's a group of bacteria called nontuberculous mycobacteria, which are relatives of leprosy and tuberculosis. And those sometimes establish in those biofilms. And so a couple of years ago, Noah Fierer and my lab and a group of other colleagues, we decided to study globally those little biofilms in showerheads to figure out why they sometimes seemed to have these bad news bacteria and why they other times don't.

GROSS: What did you find?

DUNN: So the - couple simple take-homes. One is that people who are on municipal water supply - so, like, Philadelphia, New York - that are heavily chlorinated - that chlorine seems to kill most of the bacteria in the water system, but it leaves those nontuberculous mycobacteria alone because they're chlorine-tolerant. And so it kind of creates a world in which they can live happily without much competition. On the other hand, well water from rural environments or municipal water from some of the European cities that don't treat their water with chlorine - the mycobacterium seems very, very rare. And so we're accidentally making them common by trying to kill everything.

GROSS: You mention that some of these mycobacteria cause leprosy or tuberculosis. Are we breeding those kinds of bacteria on are showerheads? Would you like to reassure us?

DUNN: No. So we're not at risk of shower stall leprosy or tuberculosis.

GROSS: Good.

DUNN: It's just the same basic kind of life. The biggest risk from these bacteria is that for people who are immunocompromised, there's some risk of lung infection and associated problems. And so the biggest take on this for those individuals - if you're immunocompromised, some kinds of water systems seem more risky than others. But there are also things people can do. For example, we're able to show that showerheads that are made of plastic seem to have less of this mycobacteria.

And what we think might be happening is that the plastic is actually degraded by the microbes that make the biofilm. They eat it. And so because there's a little bit of a food source, other bacteria that aren't the mycobacteria can start to outcompete the mycobacteria. And so in a way, what you're doing if you're using a plastic showerhead may be to feed the good bacteria that are killing the mycobacteria that you don't want. And it's still early days of understanding it. So you do have a little bit of agency, though. You can swap out plastic for metal.

GROSS: I'm still kind of hung up on the fact that bacteria actually excrete (laughter) - that bacteria excrement is on our showerheads.

DUNN: I mean, think of it as building, right? I mean, they just have this magical ability to make apartments with excretion, you know? Think of it mostly as construction and only a little bit as excretion.

GROSS: So can you assure us that it's OK to take a shower, please?

DUNN: It's OK to take a shower. This is a - it's a very, very low risk, and it's far less risky than, say, shaking hands with your neighbor.

GROSS: We - should we stop doing that? (Laughter).

DUNN: No, you should do that too.

GROSS: OK.

DUNN: We live in a world that's full of life, and we're always making decisions about how to avoid the very most dangerous things while still keeping contact with the rest of life. And, you know, I think we just have to acknowledge that we're surrounded by life, and we need some moderation.

GROSS: So a lot of people now use antibacterial soaps and hand sanitizers. Having studied all these microbes, what are they doing to the larger microbiome of our body and the microecology of our homes?

DUNN: Well - so first of all, we know that soap and water - good old-fashioned soap and water - works great to kill pathogens that show up on your hands because you shook somebody's hand who was sick or, you know, after you go to the bathroom. And soap and water saves millions and millions of lives a year. But the advent of antimicrobial soaps was really solving a problem that didn't exist. Antimicrobial soaps don't ever work any better than good old-fashioned soaps.

And in addition, those soaps seem not only to not do a better job of killing pathogens, they actually seem to favor some kinds of microbes that we really don't want. And so if you look on the pump itself of one of those antimicrobial soaps, there are often Pseudomonas bacteria living there that are tolerant of the antimicrobial soap and sort of get dished out every time you're using that soap. And so when you use the soap, you're essentially killing good bacteria and giving yourself a dose of not-very-good ones.

GROSS: What are the not-very-good bacteria that live on those antibacterial soaps?

DUNN: Well, I mean, the few studies that have looked at it very well found some Pseudomonas bacteria, which can cause a variety of infections and disease. But I'm sure if we look globally at antimicrobial soaps in different places, we're going to find different kinds of things living on them in different contexts. And so they're really a great example of where we've gone too far in trying to kill everything around us, and it's had unintended consequences. And that's a story that comes up again and again. We get scared by the idea that there's life around us. We try to kill all of it, and in doing so, we're more likely to make ourselves sick than well.

GROSS: Well, let's take a short break, and then we'll talk some more about the microbiology of our homes. My guest is Rob Dunn, author of the new book "Never Home Alone" about the microbes, insects, some of them quite exotic, that live in our homes. We'll be right back. This is FRESH AIR.

(SOUNDBITE OF TODD SICKAFOOSE'S "PAPER TROMBONES")

GROSS: This is FRESH AIR. And if you're just joining us, my guest is Rob Dunn, author of the new book "Never Home Alone" about the surprising ecosystem of gazillions of bacteria, fungi and insects that live in our homes and escape our detection. He's a professor in the department of applied ecology at North Carolina State University.

Let's talk about fungi. You say walls are full of nutrients for them. What are some of the more surprising parts of houses and apartments that fungi can eat?

DUNN: Fungi and houses are a mix because on the one hand, if you go into your kitchen and open your refrigerator, many of your food things have fungi in them that helped to make the food themselves. And so you have these sort of useful food-associated fungi. You have them in your beer. You have them in your bread. But then as you move away from the kitchen, you also get fungi that are just sort of drifting in.

And so we've found more kinds of fungi that drift into houses than there are named kinds of fungi in North America, which is partially just a reflection of how ignorant we still are about the biological world around us, that there's more fungal life in houses than we've yet - even associated with any name, much less good understanding. But then if you focus in on particular habitats in the home, you see different fungi. And so if you look in your salt shaker, there are fungi associated with the ability to live in extreme salt environments.

But the one place that fungi actually start to pose problems is in your walls. And so it's been known for a while that if your house gets wet, there's suddenly a bunch of kinds of fungi that show up kind of miraculously out of nowhere and grow super quickly. And one of those is this toxic black mold that people worry a lot about. But interestingly, although lots of money is made on doing abatement of that mold, nobody had really figured out what was going on with its basic biology until a few years ago, when a scientist named Birgitte Andersen started to try to figure out, when that mold shows up in houses, where has it come from? And we'd found that - in our studies of houses that when we look at the dust in houses, that mold is present, but it's actually quite rare. But when you - when a house gets wet, it's almost always there. And how does it ride in?

And what Birgitte knew is that that fungus is - its spores are too heavy to just float in through the window, and so that wasn't what was happening. And so she started to wonder whether it was actually coming in on building materials themselves. And so this is one of the most fascinating stories of fungi in our homes, is this question of whether we're inadvertently favoring some kinds of fungi we really don't want.

What she did was to look at brand-new drywall right from the factory to figure out, in that drywall, are there already some fungi? And she was able to show that that toxic black mold, along with several other fungi, basically come preloaded in your drywall. And so then they wait in your drywall for months or years or however long it takes for the drywall to get wet, and then once the drywall gets wet, they can eat your house from bottom to top with great joy and a little bit of laughter, I think.

GROSS: You know, there's a lot of construction going on in Philadelphia where I live. And driving past buildings as they're being constructed - I'm - and I'm not talking about the high-rises here, but, like, the few-stories-high kind of buildings. The wooden frame is sitting out in the elements for a pretty long amount of time, and, like, there's soaking rain sometimes day after day. And I always wonder, like, doesn't the wood kind of start to rot before the home was even built when it's exposed to rain like that?

DUNN: Yeah. That's a - it's a great question. For sure, species are colonizing that wood while it's sitting there, and they're colonizing the wood when it sits in the lumberyard, too. So long as it eventually gets dry, it's not going to rot very much in that short window while it's being built. But it will accumulate the spores of whichever fungi and then the - just the bacterial cells that are colonizing during that time period. And so it sets the stage for what might happen years down the road. It's a good example of why it's so hard to study these things because, you know, which fungi we find in a home might be partially influenced by what happened to the wood 50 years ago.

GROSS: Fungi - and I'm not going to say fungi anymore. I'm going to say fungi (laughter). So while we're on the subject of fungi, heat and air conditioning systems, which are moist, can breed a lot of stuff. What are some of the surprises that you and other scientists have found growing in HVAC systems?

DUNN: Well, I mean, another way to think about that question is to ask - so we often are working with dust. And so we're often trying to work sort of forensically backwards to figure out which things determine what we find living in the dust of a home. And when we look at it that way, one of the big effects is whether or not you have a dog, but another one of the consistent effects is whether or not you have an HVAC system. And so if you have an HVAC system, there's a set of microbes that HVAC systems seem to favor. And they're not species we know a ton about, but we know that we can predictably find them in houses that are using their HVAC system a lot. And when you turn on your heat or air conditioning and you smell - you know, some smell comes out in sort of burst of smell...

GROSS: Yup (laughter).

DUNN: That's actually those microbes that are set up in the HVAC system doing a little - you know, just exhaling a little bit - you know...

GROSS: That's not a good sign, right?

DUNN: ...Enjoying that change in conditions. Yeah, it's just a little breathing out.

GROSS: Is that a bad thing if you smell that, like, in your car or at home?

DUNN: So I think, for the most part, no. For people who are immunocompromised, everything changes. But for the average person with a healthy immune system, it's just another kind of exposure you're getting every day. I think the bigger issue than which things you are exposed to is how we've changed which species you aren't exposed to. And so we see far more health consequences from kids that fail to be exposed to good microbes that they need than we do from people who are being exposed to some additional HVAC microbes, for example.

GROSS: So what does that get back to? Does that get back to homes that are too well-scrubbed?

DUNN: Yeah. You know, the homes we're trying to build now - we're trying to make them more and more like the space station. And some of my colleagues recently studied this - the microbes in the space station. And it more or less looks like a - you know, a mix between a bathroom and a kitchen. It's almost all human-associated microbes and things associated with food. And a lot of apartments now look like that, too. And when that's the case, we've essentially removed all of the exposures that kids and families were once getting to microbes associated with plants, microbes associated with soil, and that transition's happened in the last hundred years really rapidly. And we're seeing a whole suite of autoimmune disorders, including allergies and asthma, but also Crohn's disease, inflammatory bowel, all of which seem to be associated in one way or another with losing exposures to those microbes we used to encounter every day.

GROSS: My guest is Rob Dunn, author of the new book "Never Home Alone" about the microorganisms and insects living in our homes. We'll talk more after a break, and Maureen Corrigan will review a new novel by John Boyne. She says it's about the cold outer limits of ambition. I'm Terry Gross, and this is FRESH AIR.

(SOUNDBITE OF MUSIC)

GROSS: This is FRESH AIR. I'm Terry Gross. Let's get back to my interview with Rob Dunn about the bacteria, the fungi, viruses, parasites and insects living in your home. He and the team of scientists he works with analyzed the populations of microorganisms living on floors and basements, on water faucets, the surfaces of TVs, in HVAC systems, et cetera. Dunn is the author of the new book "Never Home Alone: From Microbes To Millipedes, Camel Crickets, And Honeybees, The Natural History Of Where We Live." He's a professor in the department of applied ecology at North Carolina State University.

You compare some of the microbiology of homes that are, like, one or two stories or maybe three with apartment buildings, where you could be living, say, on the 20th floor, and it's harder for some of the bacteria or fungi to get up that high. So can you talk about the difference between, like, one- and two-story buildings and high-rises?

DUNN: Yeah. In general, the more removed from the world outside an apartment or home gets, the more it's filled almost exclusively with the microbes that fall off of human bodies and food. And so if you look at an apartment in a high-rise, if their windows aren't open very often, they tend to be filled with skin microbes, fecal microbes, oral microbes, vaginal microbes and, you know, here and there, some food things and nothing else, versus a home - the first floor of a home that opens its windows pretty often will have soil microbes and plant microbes, and some things that fell off a bird and a much more diverse and interesting set of species. But what we've tended to favor as we seal ourselves off is just a world where we're surrounded by our own stuff. It kind of looks like, you know, a person just dissolved when we looked at the dust from some of those apartments.

GROSS: (Laughter). Well, you write that we lose 50 million flakes of skin a day. Well, flakes - I assume its flakes of skin. And each of those flakes have thousands of bacteria.

DUNN: Yeah. So if we seal everything off, that's the default. That's the space station model, is that you're just sitting around in your own skin flakes and the microbes eating those skin flakes. And that has no historical precedent. Megan Thoemmes, a student at NC State, recently studied chimpanzee nests, which are probably a lot like the nests our ancestors once built. And she was basically unable to find chimpanzee-associated microbes in those nests. They were there but very, very, very rare. And almost everything else was environmental microbes.

And so we can imagine that that's what we used to be exposed to. And now in the last couple hundred years, we've dramatically shifted. And, like, if we look at day cares, you know, day cares are basically 50 percent to 60 percent body microbes. And there's no connection with the outside world anymore.

GROSS: When we're kind of sitting in a home with our own microbes, that includes butt and armpit bacteria. Are they especially interesting in any way?

DUNN: So I find armpit bacteria fascinating. So we actually have these glands in our armpits that are not really sweat glands. Their only job is to feed armpit bacteria. And so you have very special microbes in your armpit that historically must have had a pretty important role that we still don't really understand. And so because those glands are feeding those bacteria so much, we shed lots of those microbes around the house. And I actually think it's pretty likely that those are some of the microbes that are most important in defending us against pathogens.

If you think about your body, it's not actually just covered in skin. The outside layer of your body is totally microbial. And so, like, if a flu virus lands on your hand, the first thing it encounters is not your immune system. It's the layer of your microbes on your hand. And so I think that some of what those glands are doing may be to feed some of the microbes that best defend us from some of those things we actually have to worry about.

GROSS: Cats and dogs. So many people have pets. What kind of microbiology do they introduce into our homes?

DUNN: So cats and dogs - and especially dogs - have a huge impact on what's in your home. And so if we were to swab your computer screen, Terry, and then look at look at which microbes were on the computer screen, about half of the variation between what's on your computer screen and somebody else's computer screen is whether or not there has been a dog in that room. And so they affect the microbes everywhere in the house. Their microbes drift up and around all over the place.

And so we can actually identify whether or not there's a dog in the house just based on the microbes 99 percent of the time. And cats have an effect, too, but it's more subtle. They seem to bring both bacteria and some fungi, and they make some things more rare. And it's hard to know just exactly which effect, what sort of effects the dogs and cats and their microbes are having. And they can be very complex. But one of the things we see is that people, especially in urban environments with dogs, tend to have kids who are less prone to allergy and asthma.

And one of the things that we think may be happening is that the dogs are actually a vehicle for the connection of those kids to just a little bit of nature, that in these environments in which we've isolated ourselves so much from the rest of the world, that the dirt on a dog's paw may be enough connection to forestall allergy and asthma, at least a little bit. We don't see that effect with cats. But with dogs, it's pretty strong. And by the same token, dogs in rural environments seem to have less effect on allergy and asthma, and we think that's because in rural environments, you're getting so many other exposures to nature that the dog matters a lot less.

GROSS: With cats, there's the fear of toxoplasmosis from cat feces, especially for women who are pregnant and people who have compromised immune systems. So tell us what you know about toxoplasmosis.

DUNN: Yeah. So toxoplasmosis is caused by a parasite called Toxoplasma gondii that early on was studied mostly in people who studied cats and mice. It's a weird parasite in that it initially infects mice and rats and other rodents, and it can infect lots of kinds of mammals, but it can only have sex in cats. And so those things that it initially infects have to be eaten by a cat for the parasite to fulfill its hopes and dreams. And so early on, it was noticed that when mice were infected with this parasite that they acted very differently. They ran much faster on their little wheels and they seemed more prone to risky behavior.

And so it started to be argued that, well, maybe the mice and rats and other animals that are infected, that their brains are actually being taken over by this parasite in ways that caused them to be more likely to be eaten by cats. And it was subsequently even shown that if rats and mice are infected by this parasite that they're actually less afraid of the smell of cat pee. And so this thing that should be the scariest thing in the world to them suddenly is almost even kind of a little attractive. And so it's been shown that part of what the parasite is doing is it's producing the precursor to dopamine in those rodents in their brain and somehow rewiring what they think and do.

But for most of the history of the study of Toxoplasma gondii, that was a weird thing that happened in rats, mice and cats that was interesting to basic biologists but not relevant to humans. But then a few years ago, a Czech scientist named Jaroslav Flegr had the idea that he personally had been infected by this parasite and that it made him start to be risky. And he started to wonder if people in general, if they're infected with this parasite, that it changes their behavior. And what he would go on to show - and others have followed up on this - is that, yes, in fact if humans are infected by this parasite, they become more risky. They're much more likely to get into the car accidents. Their personality profile's actually changed. So in some fundamental way, you become a different human.

And that would be interesting and obscure, except that in some countries, up to 70 percent of people are infected by this parasite that gets to them either through their cats or through eating meat that's not fully cooked. So it's a crazy, crazy story, but it's also emblematic of when we study these thousands of species around us, they often have impacts that are far different or far greater than we could ever imagine.

GROSS: So what does that say about having a cat and how to care for the cat?

DUNN: I think it says a cat can change you. And depending on how you feel about risk, it may be changing you in a good way or a bad way. There is some evidence that this parasite can predispose people to schizophrenia, and so that's the most serious consequence of this parasite. It doesn't cause schizophrenia, but it's associated with an increased risk. And so, you know, if this is happening on a big scale, that's a pretty fundamental change in human health and well-being. And so either you can not get a cat; you can be more careful with the kitty litter. They're pretty simple changes, but we do need to think about them.

GROSS: What do you think about pets sleeping in bed with you?

DUNN: Pets sleeping in bed with you is very likely to change which microbes you're exposed to. And I think in some contexts, that's likely to be beneficial change. In some contexts, that's likely to be detrimental. And we're only beginning to understand those effects. But, you know, if your dog licks itself and then licks your face, it's transferring microbes. And there are actually some studies showing that kids who live with dogs, some of their gut microbes are actually dog gut microbes, which suggests that that transfer can be quite complete.

GROSS: Let's take a short break here, and then we'll talk some more. If you're just joining us, my guest is Rob Dunn, author of the new book "Never Home Alone" about the microbes and insects, some of them quite exotic, that live in our homes. We'll be right back. This is FRESH AIR.

(SOUNDBITE OF AMANDA GARDIER'S "FJORD")

GROSS: This is FRESH AIR. And if you're just joining us, my guest is Rob Dunn, author of the new book "Never Home Alone." It's about the surprising ecosystem of gazillions of bacteria, fungi and insects that live in our homes and escape our detection. He's a professor in the department of applied ecology at North Carolina State University. I'm going to bring up now the German cockroach and ask how that figures into your study of the ecology of homes.

DUNN: Yeah. So the German cockroach - we often talk about it as being indestructible. But it's really...

GROSS: Is that the standard cockroach, the German cockroach? Like, what separates the German cockroach from others?

DUNN: It's the standard apartment cockroach. It's the cockroach that sort of most intimately lives with us. It's the movie cockroach. It's the...

GROSS: Must we use the word intimately and cockroach in the same sentence?

DUNN: Well, how about tightly rather than intimately? It's tightly connected to the ways in which we live.

GROSS: (Laughter).

DUNN: And so we think of it as indestructible, but if we cease to exist, it would be gone instantly, too. Its weakness is us. You know, it can withstand certain levels of atomic radiation. It can get really hot, really cold. But it can't live without us. But it's a really interesting species because it's one we've tried to kill again and again. And again and again, it's outwitted us evolutionarily. And so the most recent outwitting event was the - we make these roach baits that are simple sugar baits, sort of like the - like, you know, we bait the roaches to those baits the way that we bait our kids to cereal in the morning. We offer simple sugar. They come in. In the roach case, there's poison in the baits, and the roaches die.

But a number of years ago, Jules Silverman, one of my colleagues, observed some roaches that just weren't interested in the baits. You know, you'd put out the baits, and they would just sort of walk around them. It was like they didn't care about sugar anymore. And what he was able to discover is they'd actually evolved a distaste for sugar. And recently, it's been shown that they did that by basically rewiring the receptors in their brain to perceive sugar as disgusting. And so we imagine that, you know, we're just going to kill everything, but evolution is so amazingly fast and these species so committed in some way or another to living with us that we always lose in this game. Now we have roaches that, we put out sugar, and they just don't care.

GROSS: Do you think that's because the roaches that like sugar were killed off by the bait, and those that didn't are the ones that survived, and they bred roaches that didn't like sugar?

DUNN: Yeah. That's right. So the - if we imagine that the - that roach population had a little bit of genetic variation initially, any roaches that like sugar a little bit less were much more likely to succeed. And so that's what happened. So some of those roaches had genes that made them less interested in sugar, and that lineage has prospered. Interestingly, those same genes seemed to make those roaches less sexy to other roaches. But that doesn't compensate for the - how much better they do just by being able to avoid the poison. So they avoid poison; they're less sexy; they're willing to live with it.

GROSS: So if you had roaches in your home, would you use - would you call in the exterminator?

DUNN: Yeah. Roaches are tricky. They're the one that I feel a little differently about. So German roaches - I would put out some of the new baits that they're still attracted to. There's a different sugar in them. And other roaches, if there aren't too many of them, I'd live peaceably with. And for the most part, you can control which species are in your home by which things are left around. You know, if there are little bits of food left around, it favors roaches. If you clear that food out, they become less and less common. And, I mean, the other thing to notice is that lots of these roaches have enemies in your house. And so the spiders are helping to control the roaches. There's actually a teeny-tiny wasp that we find in most houses that lays its eggs in the egg cases of the roaches, and then the little baby wasps crawl around and eat the living baby roaches and fly out.

GROSS: Do they sting? And how big are they?

DUNN: They don't sting. They're totally harmless. They just fly around your house doing good work on your behalf. And they're like an eighth of an inch long. They're teeny-teeny. And so even when they're there, you don't notice them.

GROSS: What is the most surprising thing you've found in your own home?

DUNN: When we started studying the insects in homes and other arthropods, if you would've asked me how many kinds of spiders were in my house, I would've said, well, there's the long narrow one, and then there's the chubby one. And as we started studying homes, it became clear that, no, I mean, almost nobody has just two kinds of spiders. Most houses have, like, 10 kinds of spiders. And my house was one of those houses. And so that was surprising, that here were all these kinds of spiders that were around me for a decade in my house, and I hadn't noticed them.

And Matt Bertone, who's an entomologist at the university and can ID anything, went into my house to search for all the insects and other arthropods in my house. And he came back with a giant jar. It was, like, black with life and then said, well, you've got 10 kinds of spiders, including one that's a spitting spider that doesn't build a web but just sort of walks around and...

GROSS: (Laughter).

DUNN: ...Looks for fruit flies and spits venomous silk on them. And he found one of those in the stairwell going down to my basement. And there's still one there walking around. It's tiny. It's wonderful. And I'd never ever noticed it. But once he saw it, then I could see it. But until he saw it, I was totally blind to it.

GROSS: You know how a lot of people take probiotics that are, you know, capsules or through food like yogurt - things that have, like, the good-guy bacteria so that you are creating, in your gut, a good bacterial environment instead of an imbalance of bacteria in your gut? Do you think someday there's going to be something similar to that for homes, to create a good microbial ecology in your home?

DUNN: Yeah, I hope so. It's not there now. But I look forward to a time when we can essentially garden our homes, fill them with species that benefit us and push out the species that don't. And the closest, I think, we've come to that is in the context of making fermented foods. And so we study sourdough bread a lot. And when you make a sourdough starter, that starter has a bunch of microbes that produce lactic acid. And that lactic acid is key to the starter, but it also kills off any other bacteria. And so it's kind of a garden that weeds itself. And for me, that's a metaphor for what we'd like to have in our homes - a garden that benefits us, that's beautiful and wondrous and interesting but also, in some way, weeds itself so we're not constantly going to war.

GROSS: Well, Rob Dunn, thank you so much for talking with us.

DUNN: Oh, it's been an absolute pleasure. Thank you so much, Terry.

GROSS: Rob Dunn is the author of the new book "Never Home Alone." He's a professor in the department of applied ecology at North Carolina State University. After we take a short break, Maureen Corrigan will review a new sinister novel by John Boyne. This is FRESH AIR.

(SOUNDBITE OF MUSETTE EXPLOSION'S "DOUCE JOIE") Transcript provided by NPR, Copyright NPR.

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