Teleport every one of yourselves into the next room and you'd leave a strange shadow（ [C] 影子） behind. A ghostly（幽灵的） cloud of single-celled creatures（生物） that live on and inside your body, your microbiome. Zoom in and we can see that this cloud is made of bacteria（细菌）, fungi, and others. Like these in the gut（内脏） that digest fiber（纤维） and give us nutrients we can't make ourselves or these that munch（ 用力咀嚼） our skin oils and give off that characteristic body odor. Even the film of plaque（匾） that we brush off our teeth was put there by microbes（微生物）. You're teeming（大量出现） with microscopic（用显微镜可见的） life, or rather you were.

  Without you to sustain（支撑） and contain it, your microbiome is now rapidly（快） dying. And without it, over in the next room, so are you. From your first day on earth, these microbes have helped build, protect, and feed you. And on your last day, they'll be the first to take you apart. When multicellular（[生物] 多细胞的） life arrived on earth, microbes had been here for more than one and a half billion years. They were first.

  So naturally, every complex creature to come after, from jellyfish（水母） to dinosaurs, termites（[昆]白蚁） to trees, koalas to us, has had to learn to work with them. But what happens when we try to live without them? You might think fewer bugs（小虫） means fewer diseases, but it's not that simple. Cleaner isn't always healthier. Which bug we meet and when we encounter（遭遇） it makes a huge difference in who we become. In the 1970s, a Canadian doctor noticed that local indigenous（本土的） children were less likely to get asthma and allergies than the white population, despite（不管） getting more infections（感染）.

  Later, a British doctor saw less hay（干草）-theater allergy in children who had older siblings（兄弟）. It seemed like kids who grew up in more hygienic environments ended up with immune（免疫的） systems wired to attack stuff like pollen（花粉） and household（家庭） chemicals as if they were dangerous germs. This is the hygiene（卫生） hypothesis（假设）. It says growing up around a less diverse（不同的） bunch（束） of microbes（微生物） can make our immune（免疫的） systems kind of jumpy and nervous later in life. Today, our food is safer, our water is treated, we have smaller families trading fewer germs（微生物）, and we even live around fewer animals. One scientist analyzed household dust and found that homes with cats or dogs have more varied（各种各样的） microbes（微生物）.

  As adults, our immune systems protect us by calling on a library of past infections, but when we're babies, that library is empty.