Abstract
The human body contains as many as ten times as many microbial cells as human cells, and these are organized into communities that colonize many tissues throughout the body. Each community has a different structure as the tissues/habitats provide different environments. The microbial communities provide many functions such as degradation or biosynthesis of nutrients, protection against infection, and are in constant interaction with the host. Microbial communities show plasticity in their composition, although there are general characteristics to beneficial communities. The microbiome changes after birth in ways that are becoming clearer, and the microbiome in the elderly is likewise different. An unbalanced community structure occasionally occurs and results in diseases, both chronic and acute. Manipulation of the microbiome as a therapeutic, as well as recognition of abnormal microbiomes as a diagnostic, are new frontiers for medicine.