Welcome to the weird and wonderful world of the human ecosystem. Everyone of us is a world of its own – a human habitat with the most amazing landscapes and environments: You are like a planet with 100 trillion inhabitants. These alien creatures control everything we do. In fact we couldn’t live without them. 90 Percent of the cells in our body are not of human origin. But science is still far from knowing all the members of this incredibly complex biological community. Only if we do we will really know what makes us human.