Humans in the developed world spend more than 90 percent of their lives indoors, where they breathe in and come into contact with trillions of life forms invisible to the naked eye: microorganisms.

Our bodies are home to trillions of microbes, and these creatures define who we are.

Humans in the developed world spend more than 90 percent of their lives indoors, where they breathe in and come into contact with trillions of life forms invisible to the naked eye: microorganisms.