Look in the mirror and you won't see your microbiome. But it's there with you from the day you are born. Over time, those bacteria, viruses and fungi multiply until they outnumber your own cells 10 to 1.
As babies, the microbes may teach our immune systems how to fight off bad bugs that make us sick and ignore things that aren't a threat.
We get our first dose of microbes from our mothers, both in the birth canal and in breast milk. Family members tend to have similar microbiomes.
"The mother's microbiome has actually poised itself over nine months to basically become the prime source of microbes to the infant," says Lita Proctor, director of the Human Microbiome Project at the National Institutes of Health.
But ultimately each person's microbiome seems to be unique, perhaps as personal as a fingerprint.
As the microbes colonize our bodies, they pick specialized real estate. The mouth, with all those moist nooks and crannies, is home to one of the most diverse habitats, like the Amazon jungle.
Wet places like our armpits are lush, too. But they have different microbes than those in the mouth.
The armpit microbes feast on nutrients in sweat, Proctor says, and produce antimicrobial compounds to protect the skin against harmful microbes.
Other body parts are like the Sahara Desert to your microbes. That forearm skin, for example, is dry — very dry. But even that driest habitat is brimming with microbes.
Feet have oily parts and dry parts, and it's those wet parts that the foot fungus just loves.
But the biggest habitat is the gut. It hosts the most complex and diverse group of microbes. Everything that microbes are doing elsewhere in the body, they're doing in the gut, in spades.
Diverse as these habitats are, the microbes on the various body parts communicate with each other and with our cells. Scientists have started eavesdropping on those conversations, and have started testing them as possible treatments for diseases like Crohn's, multiple sclerosis and asthma.
This research is all really new. No one knows for sure what most of our microbes are doing. But many scientists now think that if we're going to remain healthy, we have to maintain the health and well-being of the ecosystems for our microbes.
NPR has been exploring the world of the microbiome. To read and hear our series, click here.