Trillions of microbes are living in and on us, a thriving ecosystem of bacteria, viruses, archaea and fungi. Collectively these microbes form the human microbiome and this microbiome appears to play a key role in many aspects of health, crowding out harmful microbial invaders, breaking down fibrous food into digestible compounds and producing some essential vitamins such as B12 and vitamin K. New research is exploring the relationship between the gut microbiome and immunogenicity, autoimmune inflammatory diseases, neoplasia and brain development and is searching ways to correct dysbiosis.