http://academy.asm.org/index.php/faq-series/5122-humanmicrobiome
The human microbiome, the collection of trillions of microbes living in and on the human body, is not random, and scientists believe that it plays a role in many basic life processes. As Science continues to explore and better understand the identities and activities of the microbial species comprising the human microbiome, microbiologists hope to draw connections between microbiome composition, host genetics, and human health. This report from the American Academy of Microbiology addresses this growing area of research.