Ash and Mueller in Science:
In John Donne's famous words, no man is an island. Rather, all organisms, including humans, exist within a sea of microorganisms. A select few microbes cause great harm, but most are benign, some essential. In fact, many aspects of normal plant and animal development require benign microbial colonization and the establishment of specific relationships that have probably coevolved since the origins of life.
Perhaps not surprisingly, the human genes masterminding the selection of symbiotic microbes are largely those involved in immune regulation and barrier defense. In turn, the microbes that colonize mucosal tissues after birth play a pivotal role in shaping the development of the host immune system. Consequently, the effectiveness of early microbial colonization may have long-term effects on susceptibility to inflammatory diseases, such as allergy and autoimmunity. Maintaining a healthy microbiota is no easy task. Diet, severe disease, and medications can all wreak havoc on the microbiota.