Our bodies are made of billions of cells. However, the cells in our bodies are far outnumbered by the bacteria that cover our skin and inhabit our gut. These bacteria are now increasingly recognized to have an important role in maintaining our health. For example, skin bacteria help protect us from more dangerous bacteria that could invade us. Gut bacteria help digest our food. There are suggestions that changes in these bacteria, particularly those in the gut, might play a role in several diseases.