What Is the Human Microbiome, Exactly?

Are you an ecosystem? Your mouth, skin, and gut are home to whole communities of microscopic organisms, whose influence on your body ranges from digesting your food to training your immune system and, possibly, impacting your mood and behavior. What are these tiny tenants, and how do they change the way we think about human health, disease, and even identity?

The human body is made up of trillions of cells—well, trillions of human cells. Around the beginning of the 21st century, scientists learned that in fact the human body contains many trillions more microbial cells—possibly three times as many. This is the microbiome: the, the magnitude and importance of the microbiome has only recently begun to come to light.