Microbes, the immune system, and the health benefits of exposure to the natural environment
Humans, like essentially all complex multicellular animals, are ecosystems. We evolved in symbiosis with microorganisms, notably the ~1,014 organisms in the gut microbiota that provide signals for organ development, perform multiple digestive and metabolic services, and educate the immune system. Humans also evolved to tolerate certain ‘old infections’ that could persist for life in small hunter-gatherer groups. Modern medicine eliminates the ‘old infections’, while the urban lifestyle reduces our contact with organisms from other humans, animals, and the natural environment that contribute to the microbiotas, while increasing our exposure to the more recently evolved crowd infections. These changes compromise regulation of the immune system and increase the prevalence of chronic inflammatory disorders. Changes to the microbiota also predispose to metabolic disturbances. We need to restore our interaction with the natural environment because it is a reservoir of genetic and metabolic flexibility, and it fine-tunes regulation of the immune system.