We will travel from the savannas of our skin to the jungles of our scalp, from the acid lakes in our stomach to the moist caves of our intestine. Along the way, we will realize that the biodiversity on our bodies is essential to balancing our daily lives.

We harbor more bacteria than we have cells, and many viruses have taken up residence in our genes. We are now learning to harness the power of our microscopic lodgers to enhance our health in many ways, from using parasites to alleviate symptoms to managing diseases through viruses.