A fascinating book on the bacteria, fungus, arthropods, etc. that inhabit our homes and also on us. At first glance, this might look bad and your first though is how to get rid of them. But as the author shows, this is the wrong reaction. Instead, most of those inhabitants are usually harmless and are actually helpful to us as they inhabit living spaces and help deny that space to the few pathogens that could harm us. In short, having them in our homes can lead to a healthy home with few pathogens. The challenge of how to do that is left an an exercise for the reader who will probably get some ideas after reading this book.

Chapter One starts with the history of observations in microscopic life done by Antony van Leeuwenhoek. He uses his microscope to look at anything around him but it was when he was looking at pepper soaked water that he would become the first person to see the protist, single celled organisms. With time, he would see the smaller bacteria and make numerous other microscopic observations. But it would come to an end with his death as his observations are not followed up by others. With the discovery of some pathogens, the microscopic world in our homes would come to be seen as an enemy to be controlled, rather than an environment to be observed.

Chapter Two starts at a geyser in Iceland, where one of the first cyanobacterial (bacteria that use chemicals from hot springs and geysers to live and don't depend on the sun's energy) was discovered in its hot water in the 1960s. It was while studying the bacteria that scientists get the idea of testing whether similar bacteria could be found in hot environments around the house, like the coffee maker. Indeed, it was eventually found to be living in hot water heater. Since then it and other kinds of bacteria that thrive only in hot environments have been found. The bacteria would then find a role in another kind of revolution: as a way to amplify and identify DNA found in the environment using the technique of PCR (polymerase chain reaction), which depends on a protein that can operate at high temperatures that came from the bacteria. PCR would become a vital technique required to discover the numerous bacteria that live in the environment that previously was not known as they could not be grown in cultures.

Chapter Three covers the author's interest in discovering just what kind of living things can be found in our homes. It starts with the unexpected discovery of an invasive ant by the author near where he works. To discover how far the invasive ant has spread, he starts to recruit members of the public to collect and send in samples of ants for examination. For this comes the idea to sample bacteria in houses to see what can be found. What they discovered is that there is indeed bacteria in the house which can be grouped into bacterial usually found in the environment, those associated with food and those associated with our body. These kinds of bacteria can be found in various places in the house in different proportions and can even be found on the ISS (International Space Station).

Chapter Four starts with the well known story of how cholera was found to be due to contamination of water by John Snow in London. From there started the movement to remove as much 'contamination' (or pathogens) from our water. But this would involved also the elimination of harmless bacterial and other lifeforms, which outnumber the pathogens. It is only later, with studies by ecologists, that a pattern would be seen. Places where biodiversity has been reduced (like in cities, commercial farms and monoculture plantations) are also the places where chronic inflammation (like asthma and allergies) are also the highest. It is only recently that we are discovering that the price of making our living spaces so clean is that our immune system have not been sufficiently trained to detect and eliminate pathogens, causing to to over-react and to attack our own bodies or relatively harmless organisms they come into contact with. This has been called the 'hygiene hypothesis' but might be more accurately called the 'biodiversity hypothesis' and shows that saving the environment is not just good for the environment, but also for our health.

Chapter Five looks at the author's work in examining what lives in the water that comes out of our showers. In between, a brief history of human bathing is covered, showing that bathing only really caught on once clean water (from far away uncontaminated reservoirs) could be transported to cities and became a mark of wealth. And what the author shows it that water from natural uncontaminated reservoirs, full of harmless organisms, is actually the best water to use for bathing. Treated water, with less organisms, might appear cleaner, but is actually more harmful as it still contains organisms that can survive water treatment: organisms that include pathogens that, in untreated water, are kept in check by the harmless organisms. Once again, in our quest for 'clean' water, we may be causing more harm to ourselves.

Chapter Six looks at the fungi that can be found in our homes. After collecting samples from many homes, the author and his team discover not only a lot of different kinds of fungi, a number of them are adapt at living in the environment that is the home, feeding on the material that makes up our home to things we commonly have in the house. One finding that might be cause to worry is the discovery that some fungi are introduced into our homes by the very material used in building our homes: drywalls. Being dry, the fungi remain inert but when the walls become wet, the fungi start to grow and then to release spores which can be the source of several fungal diseases. This is still an area of research which may affect how we build our houses to minimise such infections.

Chapter Seven now looks at some of the more visible inhabitants in our houses, the arthropods (insects, spiders, mites, etc.). Initially dismissed as not worth the effort (not many were expected to be found), homes turn out to host a large number of different arthropods. As it turns out, people had not been looking hard enough in the beginning, which was why the study of arthropods in the house was initially dismissed as not worth the effort. New species have been found and also non-native species that may have been present in homes for a long time but never discovered until now.

Chapter Eight looks at the question that people probably have on learning about the biodiversity present in their homes: "what are they good for?" As it turns out, they can do a lot of good. Compared to the outside world, the inside of homes are places where food can be hard to get (assuming the food we eat are well protected from insects, fungi and so on). This means that insects and other organisms in the house have to get by with whatever they can find and eat. And some of the things they eat resemble some of the environmental problems we face. The author lists several examples, like 'black liquor' (the waste product of the paper industry) which, it turns out, can be handled by the bacteria living inside camel crickets found in the home as they have to deal with similar food. Other examples cited include the antibiotic used by house flies to keep their eggs clean of fungi and even new types of yeast for brewing drinks found from wasps that visit vineyards. These kinds of discoveries may only be made by understanding what kinds of organisms can be found in the home and how they cope with living in our homes. Using that knowledge, we can then see if they can be use as a solution to the problems we are facing.

Chapter Nine looks at the problem we are facing with pests in our homes. Cockroaches are one such pest and despite our best efforts to eradicate cockroaches from homes, they are still a problem. This is due to evolution: as we spray pesticides to try to kill them, all we are doing is eliminating the ones that are not immune to the pesticides. The ones who survive breed and the subsequence generation of cockroaches are immune to the pesticides being used. As the author also shows, evolution is also capable of changing what foods cockroaches prefer. Roach bait usually consists of sugar like glucose. This usually works; until it doesn't when one particular group of cockroach changes their sugar preference and begin to avoid glucose and, thus, survive the traps. The solution may lie in allowing nature to control the cockroaches by letting their natural enemies, like wasps, into our homes to hunt for cockroaches. Similar ideas are being suggested for other pests like mosquitoes and flies (using spiders). We may need to overcome our aversion to allowing them into our homes but it may be a more effective way than losing the war against them with pesticides.

Chapter Ten looks at the pets we have in our homes, like cats and dogs, and the host of creatures that bring into our homes with them on their fur or inside them. The chapter takes a close look at one parasite that has gained notoriety: Toxoplasma gondii, a parasite of cats that lives part of its life cycle in rats and, by being in our homes, humans. Research has shown that the parasite alters the behaviour of rats to make them more active and to stop avoiding the smell of cats. What is less certain is whether the parasite is also altering the behaviour of infected humans to make them more risk taking. The parasite has also been suggested as being one reason for causing schizophrenia. Research on this is still on-going. Dogs too, bring in a host of animals and parasites into our homes. But whether this is a good or bad thing is not easy to settle as while pets can be a source of parasites, they may also be of benefit to us by helping to reduce allergy responses in people living in homes with pets.

Chapter Eleven looks at an interesting point in the 1960s and 70s. At that time babies in hospital were being infected by a pathogenic form of the bacteria Staphylococcus aureus. In one hospital, a nurse was discovered to be the point of infection. Doctors studying that case made an interesting observation: only new born babies visited by the nurse were infected. Babies already a day old visited by the nurse were not. That observation, and subsequent experiments done on babies (ethics on such experiments were less strict then) would show that when babies are born, the bacteria on their mothers and in the hospitals would cover the babies. These bacteria were mostly harmless. But if they colonised the babies, then the pathogenic bacteria would not be able to compete with the other bacteria already there. Even one day was more than enough to protect the babies. That observation, and subsequent studies, would show that a healthy ecosystem of bacteria can protect people from pathogens. Sadly, this was not followed-up and the health system elected to go the path of using antibiotics to kill off pathogens (and also harmless bacteria as a side effect), leaving the people treated even more vulnerable to being infected by the pathogens which were immune to the antibiotics and did not face competition from other bacteria, now eliminated by the antibiotics.

Chapter Twelve looks at what can be done to address the imbalance in the kinds of organisms in our homes and ourselves that may be affective our health. The author now looks at a study he organised to study the kinds of organisms that give our fermented food (like Korean kimchi or cheese) their particular taste and odour. They settle on studying the bacteria that is used by people worldwide in making bread. Through the study, they discover the classes of organisms that appear in the yeast and bread dough and learn something: the bacteria also appear on the hands of the bakers who work the dough in the process of making the bread. This, of course, changes the kinds of organisms that could be found on bakers and their homes. The experiment also points out one way of 'seeding' houses and people with a range of organisms that can help us to fight off pathogens by inhabiting our homes (and ourselves) and help to make us healthier.