"Within the human body, it is estimated that there are 10x as many microbial cells as human cells. Our microbial partners carry out a number of metabolic reactions that are not encoded in the human genome and are necessary for human health. Therefore when we talk about the "human genome" we should think of it as an amalgam of human genes and those of our microbes."
"Until recently though, this abundant community of human-associated microbes remained largely unstudied, leaving their influence upon human development, physiology, immunity, and nutrition almost entirely unknown."
Human Microbiome Project, USA
International Human Microbiome Consortium