Jonathan Weiner in the New York Times:
Reader, as you read these words, trillions of microbes and quadrillions of viruses are multiplying on your face, your hands and down there in the darkness of your gut. With every breath you take, with every move you make, you are sending bacteria into the air at the rate of about 37 million per hour — your invisible aura, your personal microbial cloud. With every gram of food you eat, you swallow about a million microbes more.
According to the latest estimates, about half of your cells are not human — enough to make you wonder what you mean by “you.” Your human cells come from a single fertilized egg with DNA from your mother and father. Microbes began mingling with those human cells even before your first breath, the first kiss from your mother, your first taste of milk. And your human cells could not have built a healthy body without intimate help from all those trillions of immigrant microbes — your other half.
“I am large, I contain multitudes,” Walt Whitman declares in “Leaves of Grass,” in his great poem “Song of Myself.” But what is “self”? According to conventional wisdom, your immune system is supposed to protect you by detecting and rejecting anything in your body that is not “self.” And yet your very immune system is partly built and even partly run by microbes. “Even when we are alone, we are never alone,” Ed Yong writes in his excellent and vivid introduction to our microbiota, or microbiome, the all-enveloping realm of our microbes. “When we eat, so do they. When we travel, they come along. When we die, they consume us.”